I agree this seems like an assault, but they're neophytes just starting to explore BDSM , what if he actually really loves her and so does she but end up fucking up their relationship cause they weren't ready to talk it through. It's a cricket bat not a katana sword , I've seen worse with BDSM. It's easy for you to say , there are people having relationship cheating on their partners for years and having kids to which they aren't even their biological parents. I don't see the guy as abusive , they've had a good relationship for 3 years and they love each other , you want them to mess it up ??
I get it, I know it's not a good place to be at, but what if they end up regretting this throughout their lives cause of a novice mistake the guy did( considering he didn't know what he was doing). That's the reason I suggested her to talk it out.
If the guy is abusive as you said , she'll leave him either way after having the talk !