I'm feeling slightly sick

[Clodfobble]

I would say there's nothing wrong with being that friend, assuming you have no children. You are not at risk of being victimized yourself. And you're certainly not going to legitimize his behavior. And if everyone else leaves him, the only people he will have left when he gets out are people who would legitimize the behavior, and he will end up in their company eventually. Better to be a checkpoint of sanity for someone who needs it, if you think you're up to the task.