Cheating

I think my boyfriends mom is cheating on her husband! In the last year she has started smoking wearing makeup loseing weight and lying to us! She says she works late on the weekends and a friend says she leaves at 9 but she don't get home until 2am! She won't let anyone see it and today we went shopping and she kept getting a call from a number that she didn't have saved in her phone! When it would call he would turn it but I snuck in and looked! Everytime I tell my boyfriend he says that it is probably a friend but I don't think so! And almost everyday she picks a fight with her husband and 3 months ago she told her m she wanted a divorce! What should I do??