One of my (5 month old) pups that I bred went to a young ish couple. I vetted them as I did my other buyers, and although they worked full time it was clear to me that they had plans for this (dog walker mid morning, family staying for the afternoon with pup).

I found out a few weeks ago that they got a pup of another breed at the same time/age as my pup. This made me mad that they didnt tell me but I discussed it with them and they seemed to be coping well. I made the decision to let him stay there (well, realistically if they didnt want him returned I couldnt force it).

My friend who has another pup from same litter bumped into him today, the owner told my friend that they walk him for 45 minutes a day in the evenings and she noted how all they did when on the park (where they bumped into each other) is use a ball flinger over and over again :( and also he was very reluctant to socialise, timid etc.

I then get an email from pups owners saying that they bumped into my friend, gave an update on pup and went on to say he is ball obsessed even at home. I gave them some advice about what exercise was appropriate and how to tackle the ball obsession (fairly simple!).

I was very 'nice'  in my email as I dont want to scare them away from contacting me so to speak but I am so worried at the same time. I know there is nothing I can do getting the pup back (which I want to do). I dont really know what this post is for, I guess for understanding from people that know how I feel?!

OK, 45 mins for a 5 month old pup is too long. I'd be on the phone, not sending e-mails! Sorry, I expect my puppy buyers to adhere to my ideas, as laid out in visits and in my puppy pack. If I found they they were not, then I'd be talking to them in person.

You can't force them to listen. However, they sought you out as a breeder so they obviously thought you were a good breeder. They may well listen to your concerns. Ring them up, invite them over, or if that's not possible, just give it to them straight, over the phone.

I hate ball flingers. I hate frisbee. I hate anything that requires the 'chase' instinct to kick in. Not 100% sure but think we may have same breed. I want mental motivation rather than physical activity!

I feel really worried for you, but you're doing the right thing; just give gentle advice and remind them that you always have a place for him. I'd be livid too if I found out one of my buyers had done that! However, like you say, you can't well force them to give him back, only make the knowledge available to them. Can you meet up with them at all for a catch up to give them some hands on advice?

Thanks, yes we do have the same breed. I certainly would like to speak to them, I did try ringing but no response, I asked in the email if they could give me a call or let me know a convenient time for me to call.

I'd be carefull not to overreact on this. They are not been cruel or neglecting the puppy. They were misguided to get a second pup and have made a mistake in introducing a ball thrower at such a young age. Luckily the ball thrower issue can be rectified easily, now that you have told them it's harmful. They just sound like inexperienced dog owners who need some helpful advice and are probably doing the best they can.

They aren't taking the pup on a 2 hour hike every evening. Ideally he'd be walked for around half an hour, but they're staying in the park for 45 mins. This wouldn't be over the top if the dog wasn't chasing a ball all that time - but was just enjoying the sights and smells in the park. I frequently overstay my allotted time in the park, but my (older) pup isn't running or walking around all the time we're there. I'll stop to talk to people and he stops to sniff around or meet some dogs. We meet so many people some days it can take ages to walk round the park. I'd be mortified if my pups breeder thought I was a bad owner because of this -  for not sticking religiously to the 5 minute rule (which is after all just a rule of thumb with no empirical evidence to back it up).

The ball thrower is a mistake, but an easy mistake to make. So many people use them for their dogs and most won't think there could be any harm. I wouldn't have thought of potential damage to bones and joints from using one if my pups breeder hadn't spoken about it on one of my visits. I had no intention of using one anyway, as I'd used one with my last dog and hated that she became so ball obsessed. We all make mistakes.

The use of the ball thrower and the resulting ball obsession is very likely the reason the puppy seemed timid and relucant to socialise. My last dog v quickly lost interest in greeting other dogs or people when she became focused on the ball. If the ball thrower and balls are left at home the puppy will start to show an interest in other things again.

It was great that you kept the tone of your email friendly as I'd think there would be a risk they'd back away and stop contact if you are critical of them. Since you have a friend in the area, maybe you could say you're visiting her and her pup and ask if you could drop by and see them too. It's much easier to give advice face to face and hopefully seeing the puppy would set your mind at rest. You could then demonstrate some brain games to help them tire out the pup in other ways than chasing a ball.

Without sounding too harsh, you sold this pup.  This family handed over money so it is now their pup to do with what they will.  This is why we vet so thoroughly and place restrictions on the papers.  I stay in contact with all my puppies owners in friendly terms and I give advice but if someone really wants to do somethng I disagree with, there is not much I can do.
I would visit and see the pup for yourself to see if he is a bit shy.  We all know our dogs can be funny with somepeople and around that age.  And I would also advise about the excessive exercising but in a 'trying to save them from joint problems' way. 
This is one reason why I never sell pups to family or friends and people in my town.  The pup is too close and never raised how I would do it!lol.

I know I sold the pup, I realise there is not much I can do but advise and hope they take notice that what they are doing can be harmful to the pup. He was the only one that went to a 'pet' home. I think I will ensure all of my next litter go to working homes.

I know over exercise can be harmful and what you are saying about the ball thrower, but really I'd be more worried if someone got in touch with you to say the puppy never seen outside its own garden!  I dont think anyone takes care of the puppies quite the way we think we do :)  I've had a couple of things with owners doing or feeding stuff I dont agree with but I know they are loved and wont be intentionally mistreated so like you I bite my tongue a bit to make sure I dont lose contact with them so if there was ever any issues they'd have no reason not to get in touch with me for help. 
Homing puppies and keeping a good relationship with owners I have found is one of the hardest parts of breeding.
Be thankful this man hasnt lost interest in the puppy and hopfully He'll take on your advice. 

As above, I would be pleased that they are exercising the pup and using something that appeals to the collie's chase instincts (which might otherwise show itself in car chasing/unwanted herding behaviour etc). All they need to do is tone it down a bit because of the pup's age, which you can explain again to them. My ten collies all run on fling-rings/chuck-its and they are great for keeping the dog interested in you and not wanting to run after every tom, dick and harry they might see. If the pup needs socialisation maybe suggest they join a good training/ringcraft club as well as visits into town/markets/garden centres etc where the pup can be gently accustomed to new sights/sounds etc.