Well, as a start, I definitely recommend against telling her he's a sadist!
I like Dinged"s suggestion.
When I've had these discussions with friends I usually say something like:
"You know, I've never really been a very jealous person. And when I care for someone, I want them to have the same freedoms that I want. If jealousy ever has come up, I can usually deal with it by looking at the situation objectively and being honest. I would rather people tell me what is going on than pretend they aren't seeing anyone else. I don't want someone I'm involved with to feel like he has to hide stuff from me."