[Neo]

There is nothing wrong with it. If the boyfriend trusts her then he shouldn't get upset by it. If he does that's his own problem. Not saying you should tell him - no need to risk stirring things up unless it becomes necessary. Both of my best friends are girls (gay ones actually) and their partners know that my relationship with them is completely platonic. I think of them like sisters.

Just be careful that she doesn't start using you as an emotional crutch when things don't go her way with her boyfriend.