I have been in the position of the girl. Only difference was the man in your position was my ex. He told me how he felt and it really upset me. I cared about him but I didn't want a relationship with him and I knew that hurt him.
I tried to be his friend but all it did was hurt us both cos he wanted me and I wouldn't give myself to him. I left him alone for a long time, texted every so often just to see how he was, I cried when he told me he had cancer, sent him a card as he was recovering from surgery, Nothing to suggest we were anything more than friends. The most important thing he ever said to me was 'I would rather have you as a friend than not have you in my life at all!'
Several years later, now we are friends. He looks at my car when i have problem, He comes over for tea now and then. He as never met my boyfriend, now my fiance. He as expressed that he would like to, I don't know my fiance feelings on that event. He may have to eventually if i invite him to our house-warming and/or wedding.
I cant tell you whether or not tell her, just giving you my experience. She may realise she has feelings for you too and break up with her boyfriend in favour of you, she may get scared and run back to her boyfriend and you've lost a good friend. No one can tell you how she will react, use what you know of her personality to gauge how you think she will react which may not necessarily be the reaction you want and make you decision based on that. Good luck with whatever to decide to do.