Yes, setting boundaries is important in any relationship. If this is a work related trip, then she really would not need to tag along. He would be focused on the business/work that you and he need to do not in entertaining her.
Of course, he will also have to relate that to her (you do not want to seem like the bad guy and him to remain passive) She is his girlfriend and he needs to set boundaries or at least ensure her of whatever she needs reassurance. Neither you nor he can fix her jealousy or trust issues. Try not to get absorbed by it (this is their drama to handle) and personal affairs and work should be separated so you have taken the initiative to remind them of that.