Someone claims I have their bike but I bought this myself, how to defend myself?

I actually have a friend (he told me about this site during our discussion) who had an issue with these bike victims claiming he still their bike. They did not know the sn, and had no proof that the bike he had was stolen when it really was. I was with him wh r he got it shipped to his door are we built it together being his first high end bike. Of course local sherif department got involved, but told the victim they didn’t have the proof. I’m curious what else can be done to prove a bike ISN’T stolen?》other than register the sn here (Awesome thing going on here, put my mind at ease when I registered ), I like to prep for bad moments like this