This sounds similar to what I'm going through right now.
This customer I have only knows enough to be dangerous. He keeps messing with his amp, then calls me at all odd hours (not good when your day job is schoolteacher) to tell me his amp is "sounding" funny.
I'll pick it up, fix it, then return it. Within 12 hours I'll get a call that he thinks something else is wrong. What I've discovered is that after I repair the amp to proper function, he'll take it apart and mess with it, "just to see what I did". In doing he messes other things up. Then I get a call, accusing me of damaging his amp or not knowing what I am doing..... It's a damned Silverface Bassman, I could draw him a schematic from memory if I had to. I've since discovered that he read on a Fender Forum about "biasing" so he obsesses over it. So when I fix the amp and set the bias, he gets out his kit to see if "I'm lying". In the meantime he just stabbed in some other non-compatible octal tubes then called me at 1AM in a panic that smoke was coming out of his amp. I picked it up, the screen resistors were burnt and split, the choke showed signs of being very hot. So I replaced the whole worn out sockets and resistors (both screen and toasty grid stoppers). Now he claims the amp is "too dirty sounding"...after he took it apart and played with his bias checker toy. He sent it back to me wanting me to make sure it doesn't break up until 7 on the dial, with the master all the way up. I keep explaining that even the cleanest tube Fenders usually show signs of breakup by 4-5 on the dial, but it's falling on deaf ears.....
This one is making me want to give up my side-business in musical gear.
Here' that's my vent for people that may understand....
Doug, your service has been top notch to me for the last 10 years, never a complaint! What you're dealing with is someone out of touch with reality.
j.
This guy reminds me of a couple of my old CB radio customers back in the day. Just like this guy, they loved to stick a screwdriver into a perfectly-working, great-sounding radio. Then come to me after they screw it up beyond their ability to straighten out.
Had this one fellow who was infamous for that kind of thing. He hollered at me over the radio one day as I was in the shop (had a shop radio). The following over-the-air convo made me CB-famous in the area for quite a while!
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Him: Hey Strat! Help me out here. I've got a radio open here and I'm trying to turn the modulation up, it just ain't loud enough.
Me: You got an alignment screwdriver?
Him: 10-roger!
Me: OK, get a firm grip on that screwdriver...got it?
Him: Yeah, sure do! What do I do now?
Me: OK, take that screwdriver and jam it as hard as you can into your thigh until the urge to screw with that radio goes away. There, just saved you a doubled bench charge. You're welcome!
Him: &^!@$!&*!@%!
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Then, about thirty people all keyed-up at once laughing their butts off!

At the next local CB "coffee break" down at the local Denny's, the local CBers, including this guy looking pretty sheepish, presented me with a plaque with a "Golden Screwdriver" mounted on it in my honor. That thing hung over the shop's front counter for years until it finally closed as a warning.

Strat