I had a terrible "Billy" moment a few years ago. I was travelling and a friend said I could use her house, because it was empty. This was perfect for me, since I had a wife and kid. It turns out there was a housekeeper. She did everything for us, made us feel very comfortable, cooked, served, helped with the kid, etc. Then one day I found out she didn't have her own room, despite the place being a multi storey mansio…
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.