"Bad company corrupts good morals" (1 Corinthians 15:33), has always been the parent's most reliable come back to their children's lack of judgment in choosing friends. It's Biblical, easy to remember, and so to the point.
Of course there are times when our kids actually want to go an extra round or two on the subject so here's a question you can ask as accompaniment to the scripture above.
"What are you learning from this person?"
Now I don't mean that this question should be posed in a hypothetical sense. Really do ask and list clearly what the answer is!
It's amazing how young people are oblivious to the influence exerted upon them by others. Therefore, listing honestly what they have actually learned might help them realize how they are changing and not necessarily for the best. It might even lead them to base future relationships on things other than looks or shared attitudes.
Rich friendships are those where both individuals contribute to the betterment of the other. Let's not just "raise" our children, let's also raise their expectations of what they ought to give and receive in their search for good friends.