Leash is, once again, right on the money. Guys don't do well with hints. You tell him how you feel. He will either say "OK" or continue his behavior. Then, you decide whether to accept his behavior or end the relationship.
One thing you might want to consider, is this really that big of a deal? Are you and he serious enough that it's really important to try to put limits on who he talks to? If a guy is going to cheat, trying to set limits won't change that. If he's someone you can trust, why upset yourself? Why not become her friend too? That's just a suggestion for a possible alternative approach. Some battles really aren't worth fighting.