Help, what should he do?

So my bf just told me he saw that his mom was talking with an ex bf on Facebook and making plans to see him, first I thought that maybe they were just talking to see how each other was and things like that, nothing bad. But he told me they flirted and talked about sex and things like that. One day I was with him at his home and his mom left. An hour or so later we went to the store to buy something and we saw her parked in a street talking on the phone. Now, he told me he heard his mom telling his dad she wanted to go to another city in Christmas by herself ,it’s the city where his ex bf lives. Also she started doing exercise everyday and things like that.I keep telling my bf that it’s something between his parents, but he should tell her that he knows everything and just that whatever she does it’s also affecting all of them, he has a little sister that just turned 3 and he is worried about her the most.

I’m angry and sad for him, I can’t even see her

His mom keeps leaving the house without telling them where she’s going and he says he feels he is also lying to his dad by knowing and not telling him

What should he do?

I just want to help him but this goes beyond me and I can’t interfere in this

Sorry if my English is bad