It really depends on what he gets up to in these 'bondage group'!! Is it like a club or where they sit around talking about the latest methods?!!
Seriously, if he goes there to spectate, then fine but to put her mind at ease, she should go with him and find out what's it al;l about as it may surprise her!! If she doesn't like it then fine but communication is the key here! She needs to discuss it with him before she enters any commitment as once she's married, it could be a completely different kettle of fish altogether!
I can't believe she's even thinking about it if she's having these doubts now!