Looks like a mouse slip to me.

What to do about it?  Resign?  Draw offer?  Win?  I think it's whatever you're most comfortable with.

Just me, but I also weigh other factors, like was the opponent enjoyable in the chat and I want to play against again (resign), did s/he take the game like an OTB match (draw offer), or was s/he cutthroat competitve (win).

I would not like it either to win that way. It reminded me about a loss I had, after I had first overwhelmed my opponent completely. He could not believe it, that I had overlooked that final test of his after he did not see all of mine. Off course, I regretted the loss and still remember it, but I remember it mainly because I never had an opponent more disbalanced about a loss of me than I was myself.

Your opponent might regret his move, but will remember your chivalry and returned it. Beautiful.

What would you do in his place? How would you think when you get an offer for a draw after you first announce a mate in 8 and then blundering your advantage away? Would you think that you still deserve that draw? Would you considerate it appropriate? Or would you think that it is more appropriate to lose the game? You know the answer of your opponent, because he resigned. I agree with him and would take the win. It is a game after all.

If not for such accidents, I would never lose at bullet.