i think you should be honest with your current partner's feelings. tell them, how you feel with this other people. tell them the truth, and let them down easy. it's not fair for you to be in love with someone else, and have them thinking that you are still in love with them.
tell your friend how you feel and say it loud and proud. tell her you don't want to be used, or be a second choice from her other friend.
best wishes to you dear xx