Is it normal for a plumber to cut into drywall just to replace a toilet shutoff valve?

If you want that shutoff valve to sit flush against the wall with a clean escutcheon plate, opening the drywall is pretty much mandatory. The reason he used a push-fit coupling in the first place is almost certainly becasue the original copper stub-out was cut back too short to solder or clamp onto. You can't magically extend rigid copper inside a tiny hole in the wall without getting your hands and tools in there.

And his concern about catching somethig on fire is 100% legitimate. Putting an open torch flame into a dark, enclosed wall cavity without knowing wether there's kraft-faced fiberglass, blown cellulose, or wiring back there is asking for a house fire. Any plumber who takes fire safety seriously is going to want clear eyes on the framing and enugh space to slip a flame protector or heat shield behind the pipe. People treat drywall like it's structural concrete, but it's just chalk wrapped in paper. A neat square cutout gives him room to solder on a fresh stub-out, anchor the pipe to a stud so it doesn't rattle when water shuts off, and get everythign aligned properly. Patching a small hole behind a toilet bowl takes very little effort with a small scrap of sheetrock, a little mud, and paint. Once you slide the trim plate on, you won't even see it. Let him open it up and fix it with soldered copper or a proper **compression valve**. It's the only way you're going to get the clean finish your wife wants without leaving an awkward temporary fix sticking out into the room.

Wait, that torch safety stuff makes so much sense! But hey, if he's cutting into the drywall right behind the toilet anyway, coud you have him add a tee for a bidet seat while he's in there?!

I'm super new to plumbing DIY and have been dreaming about installing one of those heated washlets, but our bathroom only has the single cold stub-out. Does a plumber usally charge a ton extra to add a second line or mixing valve if the studs are allready wide open? Seems like such an awesome opportunity to upgrade!

Total novice question here, but does he actually have to use a torch inside the wall? I've seen videos of plumbers using those press tools (ProPress I think?) that crimp copper fittings without any heat at all. Is that something most residential plumbers have on their truck, or is that super specialized? And would a tool like that still need a huge drywall cutout to fit anyway?

Take a look at what’s on the other side of that wall first. If it backs up to a closet or hallway, I always cut the access hole from that side. Did that for forty years on tight bathrooms—slap a plastic access panel over it in the closet and you don't even have to tape and mud.

Whatever side he opens, make sure he screws a 2x4 block between the studs and straps the copper down solid. Nothing worse than a loose pipe twisting inside the wall twenty years down the road when someone cranks on that shutoff.

Quick update — I checked behind that wall like someone suggested, and it acutally backs right into our hallway linen closet! That seems way easier to hide with an access panel. My only question now: does he still need a massive opening back there if he uses one of those ProPress tools instead of a torch? Wondering how much clearance those crimpers actually need between the studs.