Oh my God, this is horrible. How depressing. I can't add much to what everybody else has said, as there has been some fantastic advice, but I really do feel sorry for you and you have my sympathy. 
Frankly, I would think she's more than welcome to him. Some men, once they get past the age of 50, become miserable moaning whinge-bags who develop one ailment after another and just moan about everything. I feel sorry for her actually. Can you imagine like, only being in your late 20s or early 30s, and being stuck with a man a generation older?
Nope. Fuck that! I know this has probably been said, but she will eventually be his carer, and probably before she is middle aged! LOL fuck that too!
If they end up having kids, SHE will be looking after kids for 15 to 20 years while she's ALSO caring for her elderly ailing partner. Plus caring for her own older parents/older family members as well. Yeah, looks like she's got a lot on her plate for the future, a life of caring and many decades of grunt work, while you've got a wonderful, single, carefree life withOUT this man who clearly doesn't deserve you.
As a few other people have said, I've known a few cases where men have gone off with younger women and have come back with their tail between their legs several years later. Some women did have them back, but others didn't, and they were never happier. They realised how miserable they were with him, and it took him leaving to realise it .
Look after yourself @HeartBrokenWife xxx