If I read it correctly, it was his friend's friendwho was doing the tickling. I guess I might be jealous about someone going after my significant other if I didn't trust them both...or at least my SO. But, unless there's overt sexual content with it, I wouldn't be too concerned.
Tickling can be sexual. Yes. But, it can also be totally innocent fun. I'd suggest that your friend talk with both and let them know his feelings if they're that strong. But, he needs to be sure why he's really upset and what that may say about the relationship.
Ann