Should I tell this girl that the guy she dating has STDs?

I know for a fact that this guy has repeatedly had the clap and has no health insurance so has never gotten treated. Also that he has . I want to tell this girl because I know he wont. But I don't want it to seem like I'm doing it for spiteful reasons. Like the fact this guy disgusts me. By now there is a good chance she already has these STDs.