Unfriending her might be taken the wrong way, but hiding her posts sounds like a good compromise. If it hurts you to see them together, you won't have to anymore, but she'll still be able to share pictures of her with her boyfriend for her friends.
I don't think it would be very fair for you to tell her not to, however you should definitely share your feelings with her, so that she knows how you feel about it. I'm a bit confused that you told him about it rather than her, I don't think using intermediates is a good idea. You obviously are in contact since she's on your facebook, talk to her directly.