Long story short on cheating GF

She lives with me, has for about a year. Long story short, about 3 months ago, she picked up some random dude with her best friend. Had sex with him that same night in a backseat and slept over. Went back a couple days later but swears nothing happened... But she slept over again, drank a lot, and took next day off work to be with him.

About three weeks later, after she and this guy texted each other over 600 times, sexting... Etc., that same best friend outs her secret to me.

She swears she was going to tell me.

Today, she's still here. Obviously much more to the story but should I in any way believe a thing she says or run.....