I love this girl but she has a boyfriend?

Sorry if it might be bit long..So now she has a boyfriend hes nepalese the girl is english im not being racist or anything just saying :) so they are together for about 2 months now but it already doesnt look like it wont last long...well to me obviously because i love her but...this guy hes my mate btw..he had 5 girlfriends before..he cheated on all of them..now the girl comes and cries on my shoulder every single time there is something wrong...she was telling me that im perfect for her,that she likes me more than i think,that im special and unique for her,she kissed me as well (only on the cheek doesnt really matter i guess thats normal ) she says im her best friend. She talks to me more than her boyfriend. She hangs out with me more than her boyfriend. When we went out it was supposed to be just us 3 and i said just go with your boyfriend because its weird for me to go you guys know why you know a couple there and than me :/ so i said just you two go and have fun :) and she goes we'll just go another because she really wants me to go...we went to a party...she danced with me instead of her boyfriend i told her to go dance with him or all 3 of us dance but she said leave him and just dance with me..we talk every single day..literally...and for the whole day...literally ! from like 10 in the morning until we go to sleep..i really love her...i dont want to let her go :/ but i dont want to ruin their relationship either help please?!!!