If she says that shes got a bf, you can counter it with "already thinking of cheating on him?" But it depends on the girl and how much youve bonded, usually shes just testing to see if youll judge her. If shes got a bf try to offer qualities that he doesnt give her, cuz usually relationships fall into a routinish-boring shit so shes looking for excitement and fun and adventures and hot sex, dont qualify to be her bf, dont offer any advice or help, just have fun.