I understand it's not your decision. So sad, but I know what you mean, I know a lot of older people who came from times when if it didn't work out they just kept the animal.
Unfortunately we aren't in that type of world anymore and you can say I have tons of money for the rest of my life, but never think that way, I lot of people who are homeless today and out of work thought it would never happen to them.
I would really try to explain to him that you will in the long run end up with the result of this union. Several people have warned you about the many things this horse could possibly have or pass on like hypp, herda, owls, just to name a few. I would test for everyone of those, especially since the reason she may have been dumped is the fact that she was hypp positive. At least test for these diseases so you aren't part of that problem. I mean would you want to be responsible for someone injured or killed because you bred an untested mare resulting in a possible positive foal? Impressive was a VERY virile stallion and has his bloodlines running through paints, appys, QH and probably others we don't even know about.
I myself will not buy a horse that isn't registered or if I don't know the bloodlines anymore. It's just too risky.