I hold a PhD, but not in physics (though I studied that in undergrad).
I advise people regularly to quit phd programs. Its low paid work, which for many/most leads to low paid post docs and then fighting for pretty low paid assistant positions, only worth it because the salary growth for the super-productive is excellent.
From your post, it sounds like he isn't super productive. So if he struggles and finishes, he will probably be able to double or triple his salary for the following few years, and hope to attach to industry somewhere. When he does that, all of his current concerns will get worse, and the PhD likely won't do him as much as the missed ~4-5 more years of real salary and benefits.
Also, a $1500/mo stipend is (depending on responsibilities) pretty low. That is much much lower than the stipend for PhD students in physics where I got my PhD. Its certainly not worth hunkering down for and toughing it out.
My advice is thus as follows: Start looking in industry for something he wants to do, or some place he wants to take his career. He seems to want to do this anyways. Once he has an offer in hand (and probably not before), he should go talk with both his advisor and his graduate director about his options.
(Sidebar paragraph for why, feel free to skip: Universities and programs face internal and external pressure to matriculate their graduate students, and to do so "on time." He is almost certainly going to not graduate "on time" so if the program director is smart, they will want the best remaining reputation they can get from him. He's not likely to publish too much at this point, or win many awards, but a high paying industry job AND graduating is pretty sweet outcome for them, far better than him continuing until his funding runs out, him dropping out and working at geek squad or starbucks.
So the program director might be willing to work out a deal for part-time status, or trying to waive any remaining lab work required or residency requirements (presumably he is long past course requirements and comprehensive/qualifying exams at this point). A productive job in industry with the ability to continue doing some data collection/experiments/lab access and finish slower is no guarantee, but it is something that most competitive programs have seen done. The minimum from those conversations should be finishing with a MA.
FWIW, I highly recommend not taking a leave of absence. Depends on the particulars of his advisor, lab, and research, but most people who take leaves of absences from PhD programs don't try to come back, but more importantly, the lab will likely have moved on without him (out of practicality and necessity). So he might technically be able to do so, but it could forfeit publishing all his work so far and functionally mean starting over, with an advisor who now thinks he is a quitter.
As for the downside of staying, the biggest downside this hypothetical: he finishes his funding (guessing thats 2 more years) then scrapes together lower funding for another 2 and then quits. He might make $65k over those 4 years of unhappiness. Then assume the economy goes bad (bloody likely, considering in the US it hasn't been doing this well this long without correction in a very very long time). So he leaves that and gets a mediocre part-time job Geeksquading it, and makes 30k in year 5, then gets a full time gig (not a real career) and makes 45k in year 6 before getting into a place where he can grow, build his career, and have retirement in year 7. So over those 6 years he might make $140k with likely minimal benefits in years 3-5.
Lets say he gets a job in physics in industry, entry level at national average or about $50k instead. Give him 4% raises/COL adjustments, and assume private sector job provides reasonable insurance and retirement but no promotions. He makes $330k over those 6 years, with an additional 10-15k free employer retirement contributions. So he stays and it costs him ~$200k now, he doesn't finish his PhD, his career and retirement have fewer years to grow. Plus most importantly, depending on the offer, if he drops out he might actually still finish his PhD.
TLDR: The economy is great, and PhD programs are terrible career moves for all but the most motivated and passionate individuals. If he can get a great offer, I'd encourage him to take it.