Trouble, i suggest you keep mute and observe them. Soon, you will hear a different story.. But sha o! They could just be innocent.
Now it is somehow bad to introduce your girlfriend to your boyfrfiend, even your boyfriend to you girlfriend. On lagosinas.com yesterday, i read a lady's discussion that her boyfriend is running away from her because his best friend molested her. what is really going going on? If i have a friend, i should introduce my babe to him/her and i do it's a problem why?
I'll advice you keep muting with keen observance but mind you that anything can happen as you are observing.
The first and formost thing to do is to give seriouse warning to her to stop call him for now and talk to your friend also that there is no relationship that will go on between them ore else............
On you own part, try to see that the girl is well cared for. Ask her questions that she will never you are going to ask her. Let her feel at home with you and be always happy whenever she see you.
Everything is not money money money without real love and everything is just sex sex sex like dog though love without sex is like tea without sugar.
The ball is in your court.............................