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Join in on some of their activities together. Get to know his friend better. Don't be passive aggressive about their relationship.
Having a chance to see if his treatment of you changes when she is around can go a long way in making sure they are just friends. If your boyfriend suddenly won't show you affection when the three of you are together, there probably is a problem. If they are really just friends, both your boyfriend and his friend should show you respect. If you can hang out with the two of them and not get any bad vibes, chances are, they are just friends and you have nothing to worry about. If you have concerns about her intentions, it might make you feel better to spend some time with them together and with her on your own. Perhaps spending more time with her can help you to see that your concerns were not warranted. When the two of you spend time together, try to view her from your boyfriend's eyes. Does she have a fun personality? Does she tell great jokes? Is she a great listener? Give her the benefit of the doubt and don't immediately look for flaws. If there's nothing more to their relationship, she should have no problem getting to know you better. It's a red flag if she seems jealous because you two have very different roles in his life. If you have trouble accepting their friendship, you may need to check yourself. Passive-aggression can be demonstrated when you are afraid to speak your mind or wary of being honest. You may not listen when your boyfriend talks about his female friend. Or, you may plan his birthday party and "forget" to invite her. Passive aggression may serve your purposes, but it can also destroy your relationship.If you notice yourself doing these things, ask yourself how you can have your thoughts and needs met in a more adaptive way.