He probably is breeding for N/H foals and on occasion breeds an N/H stallion to an N/H mare and they get the odd H/H foal, which he gave away. Lovely. That's heartbreaking because if the foal is H/H he probably won't live a long happy life.
If he's N/H and just severely affected, they'll have 2 choices. Either learn to spot the beginnings of these attacks or put him down. Having had more than one N/H horse suffer from these HYPP attacks and fall down on me, I'd not recommend keeping him. I used to groom for a lady who showed QH, Paint & Pintos and of course, being halter bred, every last one of them was N/H. Not something I would knowingly want to put any horse of mine through, for sure.