My EX druggy friend is telling lies about me,I said the dirty truth about her.What's most mature to do without an adult.Need to stop this?!?

Stop the insanity of dissing each other, move forward with your lives. And don't react to anything negative she says or does where you are concerned. When you react, it gives her what she wants (fuel to continue being a bltch)

well you must confront her in a public area , where others will hear you tell her to stop her her behavior now. or plan on suffering the consequences.