Hoping this is not spam.. For one, this is illegal, and him and his mother should know this. For second, the mother needs SERIOUS counseling. The mere fact that she even went along with the thought of video taping the act shows that something major is wrong with her, let alone the fact she pleasures herself to the video.
It is a very difficult situation that you are in. I would honestly try and talk to him, and tell him that what she did IS illegal, and %100 damaging to their relationship as mother/son. In my opinion, the mother needs heavy counseling, and then they both need counseling together.
Hopefully their relationship can be saved that way. As a friend their is not much you can do but try and get it through his head that what she did/is doing is WRONG. Because it sounds like he is very much so blowing this off. Maybe it's just a way he copes with the situation, it's not an easy one.
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Im a front porch sittin, guitar pickin, moonshine sippin,
baccer juice spittin, COUNTRY BOY FROM THE WOODS.