Have you ever had a friend change in a bad way?

I do understand people changing is a natural part of life, but seeing a good friend make really bad choices is heartbreaking. 

In the last year one of my good friends Key (thats his name don't judge) started dating this girl at my job. a few months later they broke up and the girl told everyone at my job that he was abusive and gaslighted her. Sadly many people at my job believed her, but me and my friend steve knew Key well enough to know that wasn't true. 

Unfortunately Key ghosted everyone at my job for a year until recently he called me and steve to hang out. We were initially thrilled, but we found out that our friend Key had gotten into heavy drugs and became a much meaner person. for half an hr he literally roasted everyone out our job saying things like "everyone at ur job is trash" ostracizing each person. 

I have no idea what went down, but it's sad to see someone who was once a good friend turn mean spirited. I will keep him in my prayers. any advice?