"DH is so generous hes just got in the habit of paying for her everywhere we go and taking her on holiday with us. Its getting to me."
It's always easy to be generous with other people's money. I expect his mother was grateful at first, but it never ceases to amaze me how quickly gratitude morphs into entitlement.
"Massively depleting savings" - how much money are we talking about, re her/his house? (Not asking you to state the amount here, just checking that YOU know the numbers.) The reason I ask is, when you total up money that has been paid bit by bit over a period of time it can come as quite a shock. Have you been keeping tabs? Has he? Has she? You and he needs to sit down and look at the hard financial facts of this situation. You need to know that he knows the cost to you of his generosity.
But it's not just the money, it's the time - his is now spent with her, not you.
I often think the benefit of Mumsnet is that it can be a huge sounding board. You run a situation past a load of strangers and since they don't know you or the other people they can give their tuppence-worth unaffected by emotional attachment to any party. We're not in the situation personally, we're at a distance looking at the woods not the trees. We're also coming in from a standing start, so haven't been the boiling frog as the OP sometimes turns out to be. And all we have to go on is what the OP tells us, which is often more revealing that the OP realises. (Sometimes, by noticing what the OP hasn't told us.)
Personally, I'm looking at a man whose lifestyle is subsidised by his partner (which allows him to work part-time) who uses 'joint' savings (predominantly provided by his partner because she works full-time) to the benefit of his mother and himself (since they jointly owns a property), and his subsidised part-time lifestyle also facilitates spending his non-working time enhancing the value of their property via DIY without his chronic condition affecting his DIY efforts (pretty sure OP would have mentioned if it was affecting his health) which is why he's part-time in the first place.
Those are the bald facts that have struck me.