I have a good friend and I suspect she is "cheating" with a
married colleague at her office. I put "cheating" in
quotation marks because it may not be sexual in nature.
Met up with her a few times and she mentions out of the blue
that she has a "boyfriend" of a year or so, something which I was
not aware of.
She also mentions in bits and pieces during our conversations
that there is a guy boss in her office that has got this thing for
her and that he will send her flowers with no card attached as a
present for her birthdays. But she knows it's from him. The
guy boss is married and has a young son. You know something
is amiss when girls lament that guys are all the same, with their
roving eyes and need to flirt from one girl to the next.
Why I suspect there might be more to it than meets the eye is
that she occassionally tells me she needs to go back to the office
over the weekends to work. And she apparently goes back quite
often either on a saturday or sunday. Yes, to work. To meet
up with him, I do not know.
I am her good friend and I do not wish to see her hurt in any
way.
So what should I do? 
Not girlfriend =/= cheating.
She's free to do what she want.