Some things are forgivable, or at least bearable, others aren't.

I had a friend (my best friend at the time) who lied about me to other students saying I was pregnant and various other things (I was still a virgin at the time). It became apparent she lied in order to make herself feel important. I walked away and never looked back.

OTOH my best friend as an adult screwed me over financially on a project; however, she had brain cancer and her emotional and judgment centers were damaged. It was my own stupidity for loaning her the money. Expensive lesson. I've never quite gotten over it but I love her regardless. We've been through a lot together.