Can old shower pipes be patched, or am I looking at a full replacement?

The short answer is yes, a plumber can almost certainly make the repair from the hallway side without gutting your shower tile, assuming the issue is the supply plumbing or valve. But you won't be saving that drywall with a tidy little access panel—it's alredy compromised and needs to come down anyway. When actual mushrooms are fruiting out of the baseboard, that framing and sheetrock have been soaking wet for months. The silver lining is that opening up the wall from the hallway is the preferred route for any plumber. It gives full access to the mixing valve, shower riser, and supply lines. Splicing into 30-year-old copper to rebuild a valve cluster or repair a pinhole leak with PEX or new copper is standard work and usually takes a couple of hours once the wall cavity is open.

The bigger question is wether this is pressurized supply plumbing or a failed shower pan. If the leak is constant, it's a pressurized line or valve cartridge failure. If it only gets worse after someone runs the shower, you coud be dealing with water running down behind the tile, a failed drain assembly, or a torn pan liner soaking into the bottom plate.

I'd cut out that hallway drywall from stud to stud, floor up to about waist height, right now. Pull back the carpet and toss the damp pad beneath it so the subfloor can start drying. Once that cavity is exposed, shine a flashlight in there while someone runs the shower. You'll know immediatly whether it's an easy pipe splice or a bigger waterproofing failure.

Take an old flathead screwdriver to that bottom plate once the wall is open. If the blade sinks into the wood like warm butter, the pipe repair is going to be the easy part of your weekend.

Seen plenty of houses built around '92 where the rough-in crew got sloppy with acid flux and never wiped the joints. Thirty years down the road, that green crust finally eats a pinhole right above the sill plate. If your plumber solders in new copper instead of crimping PEX, make sure he keeps a wet rag or heat shield tucked behind the torch—you don't want an open flame near old dry-rotted stud bays.

Before assuming the pipes are shot behind the wall, have you ruled out splash-out from the shower door track or curb? I've seen people prep for a major repipe when all that acutally happened was a tiny breach in the silicone along the glass frame. Water wicks along the edge of the threshold, hits the drywall corner bead, and sneaks right under the baseboard. Check the outside of that shower curb closely with a dry paper towel while running water nearby before you start planning for new copper.

Check the shower arm where it threads into the drop-ear 90 up top before you assume the lower lines are split.

In early 90s builds, guys loved tossing cheap chrome arms into a Nibco brass drop-ear with two sloppy wraps of thin Teflon. When that connection slowly weeps, water hugs the backside of the 1/2" Type M copper riser, tracks all the way down behind the surround, and pools right onto the bottom plate without showing a drop inside the stall. If the valve body itself is shot, having that hallway drywall open makes swapping an old Moen 1225 or brass Delta 600 over to a modern rough-in body maybe a 45-minute sweat job.