
This goes to the question 'for how long is any breeder responsible for the puppies they produce'? Given that environment, and the food given, plays such a big part in how a dog will be after it's left the breeder. This is the down-side of breeding and all I can suggest is you be supportive as best you can. Have they made any demands on you - asked for financial assistance re vet bills? And what's in your Sale Contract re ongoing liability? It's good, maybe, that they have let you know, but how you respond is really up to you.
I'm in a reverse situation - one of the two buy-in hounds we now have, following the end of my bloodline, has had nothing but problems, almost from day one when he came to us at 4 months, with Giardia and despite being told me had been wormed, roundworm ova. Having sorted that out, he then went lame at around 8 months, which eventually, after front to back x-rays, turned out to be premature closure of the growth plates. Over time, with one thing after another plus the latest where he was again lame and further x-rays showed his spine was basically shot, and he has subluxation of the hips, both sides, he has cost me into the thousands. To say nothing of continual worry. At one point, early on, I invited the breeder, who I'd know for as long as I'd been in the breed, to contribute to the cost of the first x-rays..... they sent me an amount which was basically derisory - and I KNOW they could have afforded way more. They didn't offer to take him back either although frankly there's no way I'd have let them take him back - when he came to us he had yellow stains on his back legs and tail. But this has left me wondering what is going on out there, this being the first time in a lot of years, I'd bought in - the last being way back when I bought my foundation bitch and then another male to enhance my bloodlines. Good job he's such a nice boy temperament-wise or he'd have been long-gone.
I hope, first of all, that you can sort out the allergies with them and can remain amicable. How many of our dogs ARE suffering with allergies these days? Is it all down to diet, or perhaps the chemicals we use on a daily basis to clean our homes?