Hi, I'd like to help you with your question.
It sounds like your friend feels this relationship works for him in some way. It is possible he feels that this fan/girlfriend admires him and therefore he can feel special and important in the relationship.
In any relationship, it is important that each partner feels respected and even admired by the other person. Being married, you are probably aware of how important it is to you and your partner to feel like you are liked and respected by the other person. When a person is well known, they may feel that it is normal to have this occur in their relationships because their lives are lived differently than most people's lives due to the natural of their work. This may be why your best friend feels ok with the relationship being as one sided as it is.
Given that this relationship is one sided, the potential for harm here is that your best friend will not be able to ever have a normal and equal relationship with his girlfriend. She is always going to see him through his fame and he will see her as subservient. Also, if he ever decides to end the relationship, it is possible that his girlfriend will react by becoming angry and lashing out, either through something such as stalking or possibly worse. She may also turn her anger on you, seeing you as a threat and one of the reasons or the reason the relationship ended. Keep in mind that these are possibilities and not necessarily what will happen. But based on what you have observed so far, the potential is there psychologically for her to act out in this way.
In talking with your best friend, tell him that you are concerned. Let him know what you have told me here, that you are fearful that this situation is not a good, solid and equal relationship for him and that it is not healthy to be involved with a relationship that is one sided. Also, mention the potential for his girlfriend to act out based on her feelings towards you. You can also express your concern that it may be difficult to continue your relationship with him if his girlfriend continues to escalate in her hostility towards you.
Hopefully, he will be willing to hear your side of things and at least take some time to think about what you have said. Do what you can to be there for him and support him. Maybe given time, he will see the situation more clearly and be able to seek out a healthier relationship.
I hope this has helped you,