Most Helpful Guy
If you knew for sure what was going on, I'd say go for it. But all you know is she spends a lot of time at his place. That doesn't mean anything inappropriate is going on...and how the heck do you know she's speing 3-4 night a sweek at your ex's house? And do you mean the whole night?
You probably are just guessing about that..you see her car there, or something?
Unless you know for sure, it would be a bad idea to tell his mom or him. It could turn out they really ARE just buddies, you know!