Oh FFS, she is deluding herself. But she's young and doesn't have much life experience, clearly.
My first husband had about 10 affairs. No idea of the length, what he told them etc, but he never once tried to leave me....and when the truth was uncovered and I left him, he was DEVASTATED. He actually said to me "I had a bit of fun on the side, but what you are doing is breaking up a family". 
I have also known several other men who have had affairs. Guess how many left their wives? Zero.
Men rarely leave their wives. Having a comfortable home life, plus sex with a GF on the side...why ruin that? They don't want to lose their cushy home life, they don't want to live away from their children (no matter how old the kids are), they don't want to leave their lovely home, they don't want to split their assets and pension.....
In 2 years time, your friends guy will invent another excuse why he can't leave, or he'll dump her, saying it's not working any more.
Best case scenario for her, let's say he does leave his wife...
- She will never be a Mum (he's too old)
- Her guy will be poor as a church mouse, after his wife takes him to the cleaners
- She'll most likely have to give up sex in her 30's, as his dick goes in to retirement
- His kids will hate her and make her life a misery (probably)
- She will be attached to a guy who thinks nothing of cheating on his wife, the mother of his children....so I'm pretty sure, for as long as his dick actually works, he'll cheat on her. After all, she's not his wife, they don't share history or children, and let's face it, he's a practiced at lying and cheating to everyone around him.
Sadly, it will be all of her own doing, and in some way she'll deserve it all. She knows what she's doing after all.