What happened to Kat's Gothic friend?

Do you mean Mandella? She got a girlfriend who I think didn't approve of her seeing Kat or something along those lines.

In episode 12 "Don't Trust Me", Kat is photographing winners of the yearbook titles when Mandella and her girlfriend "Baby" come in as "cutest couple". Baby throws a fit because Kat is taking the photo, Mandella explains to Kat that Baby is "crazy jealous" and that it's best if they don't talk until she "calms down".