Do NOT show any proof or do a group message or explain where your money comes from. It's really none of their business.
I presume you cut that "friend" out of your life in February.
If she has form for this, your friends will know not to take her word for it but might want to find out whether there is any truth to it.
Next time anyone makes a sly dig just lay it open "is that comment based on the bullshit X made up? Surely, you know X well enough to not believe some of stuff she comes out with and if you don't, surely, you know me well enough to know when someone is making stuff up about me!"
She knew full well her allegations weren't going to lead to anything. She just wanted to hurt you and make your life difficult. I would not give her the satisfaction of knowing she achieved her goal.
If it ever comes up, I would keep it breezy.
"SS spent an afternoon looking into it. Saw what utter tosh it was and moved on to actual important matters. Honestly, X should thank her lucky stars, I was too busy looking after mum to come after her for slander. Could you imagine the carnage a lie like that could have caused during my poor mum's last months."
Leave it at that. Your friends can then decide who to believe (bonus: you see who is worth being friends with).
Trying to proof your innocence will only show her how much she got under your skin.