So, he’s a guitar player with lots of money, guitars, a big Roland workstation he may (or may not) know how to use at all and “tons” of other stuff?
You’re asking a huge question here, you know. It’s really:
“Although I don’t really know anything about this stuff, how should I point this guy toward a (new to him) home recording situation with a bunch of equipment I haven’t identified yet using his unknown aptitude for it?”
AND (I’m guessing)
“I have an old MDD so why not start with that, buy the additional necessary interface(s) and other hardware to try to mate it to the much newer stuff he already has?”
AND
“Thereby force him to learn to operate the “obsolete” Mac OS 9 on vintage software and get comfortable enough on that to be creative before having him change up all of it entirely to move to newer hardware, operating system, workflow etc and learn all of that all over again.”
Sounds kinda nuts when I put it that way, huh?
Seriously… I know everybody has their own reasons for hanging on to OS 9 world - personally, I started there, got very efficient and comfortable with the workflow and I was never forced to deal with PoorTools and it works well for me - BUT I doubt I would advise anyone to start out there now - especially a guitar player (well, drummers even more so LOL) unless they have a high functioning brain and cannot afford much of anything at all so that all of the abandonware can make it all happen for them at very little cost. That does NOT seem to be the case here with what little info you’ve provided. Look… I know you know it takes a certain mentality to deal with all of the ins and outs and kinks and tangles of this old Macworld universe with it’s lack of support, unobtanium parts etc. Making someone learn all of that while simultaneously learning to use everything else and develop a comfortable work process at the same time just may be a perfect recipe for failure.
I’m a musician. I know lots of other musicians. In my too many years of experience, I have found that there is NO correlation between musical talent / aptitude and engineering proclivity. (damn, I just love to get to use words like that). Despite what Roland, Tascam, Avid and everyone else would have the world believe, it’s NOT just a piece of cake to wander into Guitar Center, walk out with some wiz-bang gizmos and software and become a YouTube star next week. It makes little sense to handicap ones progress and chances of success by starting in the unsupported past thereby making things more complicated and difficult that they need to be.
If you actually were an OS 9 DAW expert yourself and could guide him through the bumps and potholes of self-recording and using OS 9 today, interfacing it with all that he has and could save him from having to learn it all the hard way, it might be a different, workable situation, but based on what you’ve said so far, I strongly suggest you first find out what else hes has that's not unpacked yet, try to determine (as well as possible) if he's a guitarist, an engineer/producer both or neither and most importantly, how he expects to use it all and what he hopes and expects to get out of it all. Far more people fail at this than succeed and it's often because they simply start out incorrectly with unrealistic expectations that lead to early frustration with the process.