Possibly it's a way to make sure she can't form a committed relationship?
they probably treat her better than a man who isn't because he wants to keep her around
Have you asked her?
She might just be afraid of not want serious commitment and this isn't something you have to worry about with married men.
Dating a married man, you get the fun part of them and then send them home to their wife for real life.
Maybe your friend likes to play that role?
No need to seriously commit
married men like sex lol
Yeah right....stay the night........