what i do is i dont say that yeh i did it even though i didnt..... cuz i think thats jst totally wrong... when u didnt do something y r u taking the blame jst to satisfying them.....
i would tell them that "listen i didnt do it but if thinking this way makes you more happy then feel free and keep believing whatever you believe"
whereas my family is concerned well i never tell them or prove them that i have never done this or that.....
sooner or later they do find out that it wasnt me....
why are you putting so much stress on yourself getting intouch with this girl's ex just to tell this girl that no it was not you who he was after...
you should tell her once thats all.....
she is not even ur friend cuz frds dont accuze other friends for their own bad luck ..... :S