Advice; what should I do in this situation?

[thera]

Going to keep this anonymous and all, just wondering what I should do about it.

I had someone buy a YCH of mine and found out the character they linked to me they apparently share with someone else. The two people agreed the character wouldn't be used in anything 18+, but my YCH was 18+. I'm wondering if I should proceed and finish the commission(they asked it would be kept private) or maybe not even bother with it and refund them? Is it really my business? I don't really want to poke my nose in there but at the same time as an artist I really don't want something backfiring on me somewhere down the line.

Thanks a lot ahead of time. :3