I always felt sorry for Jack...
He was working on a guitar shop and ready to quit and study, when his big friend Mark called him and asked him to join DS... wow, a dream comes true, he finally get to be a rock star!
And after the last concert in Sydney, his friend Mark doesn't called him again when he was in NY doing other stuff. He not only loosed the rock and roll dream but also a friend.
Every story has two sides, I wonder why MK did that to his old friend, I wonder if anything happened between them but, with what we know, that it's Jack's side, MK wasn't nice with him in the end.
After the height of being a rock and roll star, you fell very fast to reality without any explanation. He wasn't a studio musician, but a MK close friend, and that should hurt...