What would you do if a long time friend confessed that they've always been jealous of you?

I've never had a friend tell me they were jealous of me (or even envied me for that matter). I did have a friend's mom once tell me (when he wasn't in the room of course) that she wished he could/would be more like me. I was flattered and shocked all at the same time. It was nice to hear but I never dreamed someone's mom would say that (especially to me).

Probably tell them the things I admire about them and that they should focus on what they have not what they don’t.

Lol must be a serious failure if he’s jealous of me, there’s nothing to be jealous of!

I wouldn't care. I have no control over another person's emotions. Been in that situation before.

I'd probably just say, "sorry" and awkwardly try to change the subject.