maleficent

Invite her out for coffee.
Tell her what you said here.
Write it down first, so that you don't get emotional, and don't forget what you want to say. Or just write the letter to her. If she's familiar with computers, show her his internet usage history.
It doesn't matter who taught him how to do it -- the fact that he's doing it should be the concern.

That he gives you a bad feeling, and you are concerned for the children. A mother would appreciate that.

If he's living in the weight room, then she must suspect something as well.

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