In a pond full of gear fish this may be heresy but if your pal can't get a decent sound with what he's got throwing money at different gear probably won't fix much. Taking some lessons with a player who can show him how much tone control is in the hands might.
The "in your hands" comment is actually spot on with "decent" gear. The worst piece of

I ever played through was a GK Backline 250 into a GK115bpx. That cab was impossible. nothing you could do with it could get rid of the boomy and muddy sound. It was not capable of discerning how you were fretting the notes (pad or tip of fingers), where you were plucking the string; or how you were handling your muting - nothing sounded good out of that. I was shocked because I'd actually used a GK guitar amp into a couple of JBL 12" with full width horn cabs, that sounded really good at one time. The rest of GK's gear may be great, but that "Brat Pack", as they marketed it, was terrible.
Plus it had an 8Ω minimum load so you couldn't add another cab to it to improve the sound. I got around that by hooking a second cab up in series and that second cab, a Sunn B202 (212 cab) helped get some clarity. Then I got an Eden Nemesis RS400 and at least the amp itself was putting out a clean, clear tone, but it was too much for the Sunn cabinet (only 100-watts rms handling power). When I added an acoustic B410 cab to that GK115bpx, AND cut the Bass EQ down to 10 o'clock, then I got a decent sound out of the rig. And THEN, my hands made a difference.
The reason I kept buying better and better gear is because back in the early 70's, when I was making a good living touring for 3-years, I had the best gear I could get at the time. Then in 1976 it was all stolen from me in a house burglary. Went without any bass gear until 1985 when I bought the bass in my avatar, a 1982 Jazz Bass with the same neck shape, profile and radius as my 1965 Jazz bass that I bought in 1966. In 1997 my son gave me his old Yamaha 115 combo and I could gig again. Then from 1998 until 2005, I couldn't gig because of crazy work schedules. Started gigging again in 2006 and continued buying better and better gear from then until 2021, when I bought my last amp, a DNA-800 that David Nordschow built for me from parts he had on hand because his factory was waiting on parts. I bought all of that with money made by playing music and I'm not about to apologize for that to anybody. You want it, you work for it. You work for it, you get it.
If you have decent gear, your hands do make the difference. If you have gear that can capture all the nuances your hands can supply, that's even batter sound. I used that darn GK115bpx as part of my 410+115 rig for 5-years because I could get a decent sound out of it. I use my DNA and Eden gear because it has the sound I hear in my head and has the clarity and articulation to pick out and project everything that my hands are doing. I didn't let that Yamaha 115 stop me from gigging, nor did I let that GK115bpx stop me from gigging. I got the best I could get out of it but I knew I needed better gear. So I worked and gigged and got a DNA endorsement after I'd been using their gear for 4-years. I continued to "buy better" until I was pleased with the sound I was getting and had the appropriate gear for all the different venues I would play.
I'm actually agreeing with you that hands make the difference in tone, rhythm, and articulation as long as you have decent enough gear to hear it. Not necessarily the best gear, but decent enough. Speaking from experience, it's really hard to put up with

when you've had the best, at least the best at the time.