I lost a friend to a lie that someone else told.?

Leave it alone... or confront "both" at the same time. If either one "doesn't want to", tell him/her that "you" need this! That you need some kind of closure. And even tho you're intentions are to befriend Abby again, I'm sure getting closure will come with that (the mending of the friendship)... and if it doesn't, at that point & time let her know you still want to be friends. Who knows? Maybe it "is" just closure you need! But you'll never know unless you give it a try. And if neither party is willing to talk to it out with you? Then neither one is your "friend". That's the bottom line. And there are no other logical solutions to your dilemma. So give it a try, for your own sake & sanity, and give it a rest after that. Good luck.