I'll say this. That first girl has a boyfriend already, so if you really do care about her, maybe it's best that you stand back, don't get in the way, and let those two be what they will be. I won't get into too much detail with my own personal life here, but I was in a similar situation...and find myself in something of a similar situation now (except the girl in question and I actually have a history together). 

Bottom line being, she's seeing someone and if said someone makes her happy....maybe you should let them be happy. Would be different if it was just someone she liked but wasn't actively dating, but if they are dating...maybe just try to let it go, hard as that is to do.