I've got to say that slavery as a moral ill outweighs social awkwardness, and rationalizing the situation by saying that maybe it gives slaves' life meaning is an abdication of moral responsibility.

I mean, yeah, you could find meaning in that. I'm pretty stoic, I can find meaning in things like dying of cancer or whatever. But that doesn't make it acceptable to put someone in that position, since there's an implicit assertion that their life would lack meaning without this structure.

I think you should swallow your anxiety about this and discuss the matter with your friend, who essentially coopted you into participating in an abusive relationship by not informing you of this in advance.