(((((Julia))))) It's such a tough call. In a way, one could say it isn't you business, but it certainly is. But even that doesn't make it clear what you should do.
It could be that your mom knows, and is choosing to follow this path, and that you saying something might upset the balance that exists. Is that a good thing, or a bad thing? I don't know! If he is as easy to see through as what you've seen, I would really suspect that she knows, but then again..... You were "out of the loop" for a while, I think it is possible that they came to an understanding, spoken or unspoken, and you saying something might throw it off.
I think I fall on the side of thinking that it is between them, though I think I might tell him what you know/suspect, and encourage him to "do the right thing", whatever that is. I'm not sure you should tell her. But then again, I'm not sure you shouldn't!
