What does your DH say?
Get him to talk to him 'man to man' and explain some of the diseases and reputations he could get sleeping around. the rumor mill can do alot of damage if someone wants revenge If she or to just cause trouble.
If people are warning you about her it must be pretty well known what sort of girl she is. Does he know about her reputation, if so, what are his thoughts about being associated with someone like this.

It is hard having teenagers and coming out the other side sane.

Ask him if when he marries does he want someone who has a reputation of sleeping around. If not, then he knows what his actions should be. You cannot sleep around then expect to find someone who hasn't....double standard in my books.

All the best dealing with this. xxx