That is a sad one. To us the bdsm life style does not seem that extrema. But to the uneducated vanilla world we are the devil incarnate.

I have the XM radio on the bus and I pas my time listening to playboy radio. (of course not when the children are on board or when others are near by. would Not be a good thing)

And every once in a while they cover porn addiction and the impact of an other wise vanilla relationship.

The teary eyed wife will call up fresh from her divorce crying about how she found porn on her husbands computer. Promptly moved out would not take any calls from him. Not allowed to see the children etc etc.
He is forced into consoling and the marriage ends in divorce.

He people were guys, Guys look at porn. Speaking as a probably porn addicted person. I would think that what should have happened is he should have sat down and talked over what she had found and she should have been open to his needs. But no she was a Mormon bible Betty screaming to Mommy as soon as she saw a tity.

Now as to your friend. I would think that it is not a good idea to be in a vanilla relationship but secretly crave BDSM or any other unknown kinks. I would think that it will always end badly.
I speaking as truly unqualified to give any advice since I have no relationship. But I would think that openness and communication should be there first before any kinky endeavors happen.

any way that is my 2 unqualified cents worth.