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Hello and thank you for using JustAnswer. I think you do have a legitimate issue with your girlfriend not telling you that she is inviting this other male friend to these family functions because if the situation were reversed she would want to know if you invited anyone to a family function in her place. Most likely though you can trust her as this is just a friend, unless your girlfriend has given you any reason to not trust her in the past. Going to these family functions with a friend is perfectly permissible, but she should not keep that hidden from you and she should discuss it with you for the sake of transparency.