After having two daughters who both had bad first marriages, I certainly would not hold my tongue now! One son-in-law was a serial womaniser. But we didn't quite believe my daughter, thinking she must be imagining it, or exaggerating. We never interfered, or confronted him. (Oh, if only I could turn the clock back!)
The other son-in-law was an alcoholic, who was always insulting and nasty to my daughter - but I just stood back, hoping everything would turn out OK (because she loved him). (And, I think, because I was too wimpy and didn't want to cause a row.)
They both ditched these men (eventually) and luckily have now married two lovely, faithful, sober, sons-in-law.
But I heartily regret my standing back, and biting my tongue - which equated to not supporting my daughters. Never again!
Of course, we all learn from our experiences.
If I were in Confused2's place I would certainly speak up and tell this son-in-law that his behaviour is unacceptable. Of course, she has to be prepared for her daughter's reaction. But I think we have to tell the truth, at the time, and not be frightened by the consequences.