You guys bring up a lot of good points. If he was a friend that I only visited rarely, and could stand to lose, then I would do it without hesitation. But, given how much I value the friendship, I'm not going to rush into anything, as it's obvious the negative possibilities outnumber the positive ones.
Now that I think about their relationship more and observations I recall from hanging out, it seems that he may be a slight dominator type, and she may be a slight submissive type. I think he may enjoy her submissiveness, and he may enjoy the idea of sharing that submissiveness with his friends, much like someone with really good drugs likes to share them with friends. This is just speculation, but that's what my gut tells me so far.
I forgot to mention: although he's admitted to letting her have sex with friends, there is no indication that he's had sex with other women (the openness could be one-sided). He does have some physical problems that might prevent him from being a good lover, so it could just be that he wants his girlfriend to have fun more often, and at the same time share her with a good friend out of generosity. I've even seen her cuddling with another friend one time recently.
