You have done what you can and made a very very generous offer. They have refused for whatever reason when I think they ought to be on their knees grateful. They certainly do not sound like a place you would want to send your nice mare so good on you for declining. There are many ways to raise an orphan foal and the non nurse mare way is very labour intensive and I have done it twice. If they are that casual about their care I am pretty sure they are going to find hand raising a big chore that they may not do well and thta will impact on the wee one. Nothing you can do…people make choices.