Should I tell my friend that her girlfriend is not loyal to him?

Should I tell my friend that her girlfriend is not loyal to him?

I have a guy friend and his girlfriend is also my good friend. while she is in relationship with him, she goes on dates and texts with multiple guys (he doesn't know about it). he loves her and is very loyal to her but she isn't.

Should I tell my friend that her girlfriend is not loyal to him?
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