Unfortunately, I've heard the "I was drugged" story often, especially from younger girls. That being said, it happens, so it could certainly be true. As tadhealth said, get her tested. That way a) you will know the truth and b) (if she was drugged) she will get a much needed health check.
Now, if she was drugged, then your buddy needs to be there for her and forget about him and their relationship for a bit. Her mental and physical health is the first concern.
But, if she was not drugged (which I suspect is the case), then your buddy should look her in the eye as she is crying (they always cry) and say something along the lines of "I'm sorry you are a weak person". Then walk away and don't ever speak to her again.
Normally I wouldn't advocate kicking someone when they are down like that, but the one thing I have ZERO tolerance for is a female who cries rape. These are the filthiest pigs on earth and deserve a fate worse than death. But if your friend got all mad at her and yelled or whatever, it would accomplish nothing. Just walk away. Maybe turn around as you're leaving and spit on the ground.