alcohol and/or drugs while working, even a single time in their lives, would be fairly low. The lucky ones survive addictions and get the help they need and don't relapse.
When did your friend tour with him?
Mick Taylor was the best musician ever to join the Stones. In return he got drug addiction, little of the songwriting credit he deserved, and years lost when he'd've been much better off finding a great frontman he could also write songs with, someone like a Robert Plant or Paul Rodgers - someone who was as good a singer as he was a guitarist - instead of being brought in to help the Stones when he was never going to be treated as an equal. The Stones shafted him in 1982 when they switched record labels and cut him out of royalties he was owed - a deliberate choice they made, and one that really hurt him. The Stones signed what was then the biggest record deal in history, and cut Mick Taylor out.
He's far from the only musician to get ripped off - by bandmates or managers or a label. But few musicians would have had as many reminders of that over the years as Mick Taylor must have had.