Have you told her your thoughts about the issue? I guess if she is persistent about it you might have to end up being quite firm about not being interested, explaining why it makes you uncomfortable and that being in a relationship your feelings have to be respected. Basically try to put an end of the discussion by putting your thoughts on the table and you might end up finding it could go one of two ways. She respects your decision or you both might think it's best to end the relationship.

You might want to ask her if she has feelings for him, too. To be honest, I've heard of similar situations before and it invariably ends up being a precursor for them starting a sexual relationship.

I hope it works out for you.