Have you ever watched in wonderment as one of your female friends crosses the line with the guy you are dating?
Here's what I know...
(1) You need to be honest with yourself and decide if you are an overly jealous and sensitive person or if it is obvious that your friend is crossing the line.
(2) It would be appropriate for your friend to say hello to him at a party and having a 2 to 5 minute conversation with him. It would be inappropriate for her to corner him for 30 minutes with her "come hither" eyes batting away.
(3) It would be appropriate for her to include him on a group email or an email to both of you. It would be inappropriate for her to email him directly and asking him a question about the two of you that she could have just as easily asked you directly.
(4) It would be appropriate for her to let him buy her a drink. It would be inappropriate for her to assume that she is "his second girlfriend" and that he will buy her dinner every time you all go out.
(5) If you do have a friend who is crossing the line in this area, you need to think about whether she is really your friend, your frenemy or just after your guy.
(6) If you think that your friend is crossing the line and you are not certain that she is doing it on purpose, have a conversation with her. She might learn something about proper behaviour and you might save a friendship.