hey - i had a threesome a few years ago (before i was married and pg!) and i didn't regret it for a moment. it was a great laugh, with two people i liked and respected a great deal. personally i would be VERY wary of having a threesome with a man i'd never met.
sex isn't ALWAYS about love but IMO it should always be about mutual respect and also balance. i could warble on about cosmic harmony but i won't.
at the end of the day it is your friend's choice, and all you can do is be there to pick up the pieces afterwards. HTH.