After my grandpa passed away, it wasn't too long (though, to be fair, it was more than a few months) before my grandma started dating again.
It wasn't that she hadn't been completely in love with her husband, or anything like that. She was just really lonely. She'd spend the better part of 50 years with my grandpa, and they did everything together. Before that, of course, she'd always lived with her own family. She'd never in her life had to live alone, and it made her really unhappy. She told me that 'single life is fine for people who choose it, but I never chose it.'
Now, a few years later, she's got this man living with her. No one in the family can stand him. He's just horrible! Rude, racist, sexist, ornery... just everything my grandpa wasn't. But my grandma, apparently, loves him. And, more to the point, she's got someone to take care of again. She's got something to do with herself. She's got someone to go fishing with. She doesn't have to do stuff alone. We all dearly wish she'd have found someone more suitable, but we're glad that at least she's happy.
So, maybe your FIL just couldn't deal with the idea of being alone. That can lead to people making hasty decisions.
Of course, on the flip side, my other grandfather's health was failing, due to a series of strokes, my other grandmother ran off with a man she met at church. Divorced my grandpa, moved half-way across the country, barely spoke to any of us for years. It's getting a bit better now. But some people are just really selfish, and also, they can't deal with illness/death. And that makes them do weird things.
Sorry things are so hard on your wife at the moment. Hopefully, in time, she'll find out it's not as bad as she imagines. But she shouldn't feel pressured into accepting the situation either.