All we have to give is our experience, strength and hope. Does your friend want to get back on track?
There are no magic words, just you being honest about your own experience, and only if your friend is interested in hearing it.
I hope you soon have your sober support buddy back. It is SO hard when someone we know in recovery takes a tumble, when they were someone we looked at as an inspiration. We are all human, and need one another.