My gorgeous gf and her best friend have romantic chemistry. do i need to be worried?

oh..god..! why are you allowing her with him...???first tell him to be in his limits....and also you take her to movies,pubs and swimming....and also discuss it with her seriously....... where there is no faith there is no love...if you feel that she is good then believe her if not its better to leave her...i hope she loves you but she goes with him like a friend or sister...! Good luck..:)