I think it's totally weird he's asking about your friends to begin with. I'd tell him "Before I had a chance to tell her about you, I found out she was interested in someone else so I think it's not the best time to mention it"
That way he knows you never said anything. I could see it maybe being weird if he thought you told her and you didn't.
I would do due dilligence and say something to your friend..."Friend, FWB was talking to me and wanted me to see if you would be interested him. I know you aren't but thought I would just throw it out there." That way you can go back to FWB and let him know she is talking to someone and if that doesn't work out then maybe.....even if you know it never would happen.
I think it's totally weird he's asking about your friends to begin with. I'd tell him "Before I had a chance to tell her about you, I found out she was interested in someone else so I think it's not the best time to mention it"
That way he knows you never said anything. I could see it maybe being weird if he thought you told her and you didn't.