I'll admit, I was coming here to ask because I knew the answer would be "Grad school? No way!" Now how to avoid saying "I told you so..."
So my advice would be to try to establish himself as a consultant. It is a less stable life but can be plenty lucrative, and goes along well with the concept of lifestyle design. It also dispells the idea that "rising up the ranks" in a company is supposed to be desirable. Some people like being an engineering manager, but for someone who wants to do technical work, "progression" only gets you further away from a rewarding work day. For a consultant, rising up the ranks means doing more interesting projects for higher pay.
We continued talking and ended up in a similar headspace. Friends (who have been in *other* consulting industries that get shat on, like graphic design) have warned SO away from contract work, so it's good to hear the paradigm is different in tech. My uncle has offered contract gigs to him before, so it'd be easy to transition, too.
The words "retail giant" told me why he's bored. At any large company there is going to be bureaucracy and mundane business problems that programmers need to solve. So one step might be to find a job at a smaller company.
Second, I have found that it's not always about the work. I've finally found myself in what would have been my "dream job" after 10 years of working in bureaucratic organizations. I have a lot of autonomy to solve problems using whatever technology I feel like learning and using, and can use a bunch of techy buzzwords to describe my work. But I'm bored. I don't really have a team, there's not much enthusiasm in the organization for what I do, and people I work with aren't very progressive about adopting a new way of doing things. Looking back I was actually happier at my first job because the organization cared about my contributions, rewarded me for it, and I was constantly learning and growing my career.
At this point, if I could go back, I would have studied for the interview and gone to work at a FAANG company and be FIRE by now.
A lot of this rings true to what I know of him. His old job wasn't perfect, but he had a team and higher ups who cared about his contributions and growth, even if small. He ended up leaving due to a micromanaging manager (theirs was the only team with 'core hours' - which stretched from 10am to 5pm) and a direct grand-boss who wouldn't let him switch teams. Now he has a better manager but his days are spent doing tickets to clean code, being told "we don't have time to clean that code, throw it all away, clean code isn't useful anyway", and banging his head against the wall at the hypocrisy. Next try is to switch teams, which might help. His old job has been ringing him to come back, and he's gotten really close to doing it, but it'll probably only be marginally better than before.
TBH I think he just needs to get out of the giants. He was won over by thought that a FAANG company would have great opportunities for growth, and turned down a startup job for it. Maybe 5 months and 1 team is too early to tell.
I'll second this. I don't have a PhD, but I do have a couple of Masters degrees. Depending on the subject/program, it can be grueling. In addition to the time demands, academically oriented grad programs are designed to remind you at every possible moment how little you know and how undeserving you are of working in that field. They're as much about indoctrination as they are teaching you research skills or a body of knowledge. Read Disciplined Minds by Jeff Schmidt for a very interesting and (in my experience) accurate argument about the "professions" as class gatekeepers.
I'll pass on the book rec. He's recently discovered the library and has been reading things from quantum physics and political histories to business and soft-skill self-help books. He never read much before, I think he's making up for lost time.