It doesn't matter what "power" his Mum has, she can't argue with the law. You are entitled to half of everything, and also half his Pension (and he yours).
My first husband did exactly what you describe. It's horrendous. I gave him every opportunity to change, but he just kept on doing it. After a while, I had suspicions that something was going on between him, and my very best friend who was going through a divorce.
He vehemently denied this. When he was occupied with something, I found his mobile switched off in a drawer, I powered it up and there were the text messages between them, confirming they'd had sex.
This was 4 years after the initial betrayal (well, me finding out). I left him. He was never going to change.
He cheated on his next long term GF. She left him. He's now cheating on his current GF (who he's been with for about 4 years). Leopard. Spots.
Also, (and I realise this sounds bad), but I was really pretty (getting on a bit now, ha ha), and the girls he cheated with were not. He was getting sex at home. I've no idea why he did it.
Anyway, I digress. I think you should leave him. And tell your "friends" husband exactly what you saw. The kiss, the footsie. Everything. What are you protecting her for?? Also if you have access to any texts or e-mails between them, forward them on.
I'm really shocked that you continued to be friends with her after the first kiss. Why?????