My sister has a one year old Labrador that is a sweetheart, lots of energy and very loving. She has been spayed about three months ago. My sister has recently got a male cocker spaniel who is on the small side, he’s 15 weeks old and he’s starting to become quite aggressive towards the older dog.

He’s been around her nearly a month now and they are great company for each other. He follows her around and they love to play, however I think as he’s starting his teething, he’s taking out on her. He will bite down on her stomach, neck and mouth quite hard where she’ll yelp loudly. She doesn’t seem to be fighting back, but sometimes she’ll lean her head on him and he’ll yelp because she’s squishing him.

It’s got the point where she’ll yelp, walk off with her tail down — practically in-between her legs. I’m worried he’s going too far and she’ll feel bullied in her own home.

A bit of confusion on whether to interfere and give him time out or allow her to tell him off and teach him manners — I want her to feel that she is being protected and not unloved.

I think, fwiw, that enough is enough and if this is not causing her to yelp and walk off with her tail down, YOU surely have to wade in and correct the puppy - unfortunately from my experience, whereas entire bitches will deal with males, all too often when spayed they won't.   If this is to work at all, and for them to bond, I think now he's getting rough with her, you have to be involved.   Ideally if the older dog tells off the younger pushy dog, that's better than you doing the telling off, but do you want her to be upset, distressed and perhaps eventually injured?

I wouldn't!!

Is she socialising him with other adult dogs to learn appropriate manners?

He isn’t allowed out for walks yet — he’s in the process of having his jabs, other than the Lab — No.

The adult needs its own space away from the pup. Babygates and crates need to be utilised. When the pup is being a pain, remove and give the pup something else like a filled kong,  basic training in another room or garden or even sleep if it needs it.
Pups needs things to chew which are appropriate and another dog isnt it and neither are humans lol

>He isn’t allowed out for walks yet — he’s in the process of having his jabs, other than the Lab — No.

You have a 15 week old puppy that hasn't been socialised with any other dogs or gone out on the ground in public???

What is the world coming to?

He really needs to be out meeting other adults and learning manners now - socialisation is far more important because far more dogs end up rehomed or PTS through behavioural reasons and often lack of socialisation is at least part of the reason for that.  If he doesn't learn manners soon, it will be MUCH harder to teach them later as he's learning to bully other dogs and that it's a fun thing to do because it gets a reaction (which from her, is not severe enough to stop him).  You do need to be stepping in and teaching him yourself - not doing that will do just as much damage in the long run - but he really does need to start meeting other adult dogs.

IME these first 3-4 months really are the most important stage for setting behaviour in all areas of a young dog's life.

Oh dear. Good luck with that