Big relationship problem.... HELP?

ok i have this really big problem... its sort of a relationship i guess ! so ders this guy who's in a band and hes pretty good friends with my friend dey talk alot on the phone and stuff anyways i neva really use to talk to him but i kinda as a joke became like his groupie but as time went by i realised how sweet and nice he is ! anyway... i sleep with him alot ! were not together or anything we just... sleep together... its kinda complicated. i always new my friend kinda liked him but now i found out shes in love with him ! if she finds out im sleeping with him she'll be devastated and shes my best friend, i don't think i love hm, i definitely don't love him the way she does ! shes crazy about him i don't no what to do because shes shy and kinda old fashioned so shed never tell him how she feels and he has no idea so if i stop sleeping with him it wud b pointless because she wont tell him and i don't think shes his type anyways, but if she finds out were sleeping together i think I'll loose her and shes my best friend ! what do i do ! pleaseee help !