Actually, at the time, Mike Starr had cleaned up and was the person who heard Layne's last words spoken to anyone. "don't leave me like this man". This was after they were in an argument where Mike was trying to make Layne go to rehab.

You're absolutely correct, but that addiction spiral is one that has ebbs and flows. Layne and Mike were the two who shared the same orbit apart from Cantrell. I've always seen them as two bright but similar stars (no pun intended) collapsing into the same demise. You never really escape heroin.