We all have different methods. Here's my recommendation: Tuck some tar paper (or some other barrier) between the copper and the existing metal lathe if you can. Wrap the tub stub with tar paper, tape it in place. Doesn;t need to be anything purdy or fancy. You want to keep any cement-based product off the copper. Patch in new expanded lathe over the tar paper, you can weave it in to blend the new metal with the old. You'll want to mix two things up; a portland cement slurry, and fat mud. Portland cement bonding slurry. This will act as a bonding agent to help the new mud stick to the existing wall mud. Simply mix portland cement with water to make a thin paste. Brush it on the existing mud to help the new mud stick to the old. Pretty simple. Fat mud. You'll fill that gap with fat mud. "Fat" simply means it has lime in it. Lime make a bit sticky. Recipes vary, but generally, mix by volume 4 parts sand, 1 part portland cement, and 1 part lime. You can buy bags of each individual component at most any box store. Dry mix all the ingredients. Add water carefully. You mix it wetter than deck mud (deck mud is wetted until it's barely moist, it's grainy like beach sand used to build a sand castle) but not as wet as a concrete mix that can flow out when poured on a flat surface. When shaped into a mound, fat mud will hold that shape instead of slumping flat. The goal is to get the fat mud to be wet enough to stick to itself, but dry enough to stick to vertical surfaces and not slump or fall off the wall. If you do youtube seach for "fat mud shower wall" I'm sure there are videos out there. When ready, clean existing surfaces, then paint them with the portland cement slurry. There's no magic here. Just get it on there. Then fill your hole with the fat mud. You can pack it in with a trowel, build it out so the mud patch s slightly proud of the surrounding surface. Then take a stick that's wider than the patch, place it against the good wall below your patch, and use a left/right sawing motion to shave the mud flush with the existing wall.

Grout (with the possible exception of epoxy) is never waterproof...sealed or not. Drywall is susceptible to moisture and, the "greenboard" was used in years past on tub and shower walls. It was touted as "water resistant" because of a waxy covering on the face...but was about "two spits" more resistant than standard drywall and is no longer allowed for this purpose. Regular drywall can be used in this application if it is covered with a Surface Applied Membrane such as Kerdi. I doubt this was the case here. The minimum that should have been done was to staple roofing felt or plastic over the studs to prevent vapor from transmitting to the studs and damaging them. By removing the cover I mentioned above and using a flashlight, you may be able to determine this, and what the backing is. The differences between drywall and cement board are pretty obvious. Will the job last? Anyone's guess...but it lasted three years so far, so it could last 20 years...or it could fail in six months. What you do have working for you is the tile size. With that size, there is less grout areas for moisture to penetrate. Keep an eye on the caulking at the vertical and horizontal tile junctions and use a pure silicone caulk in those areas. If they were grouted, that's not good. It should be removed and caulked. Another tip is to towel the tile down after use. Keep it dry and it will make a difference....if you can convince the renter to help you out here.

you can clean it out well id use diluted bleach and scrape all the loose stuff off,vacuum /dry it out. go back to home creepshow and buy a small tub redgard and paint it in there with a chip brush and use mesh tape (as long as the backing is solid in there). slather it onto edge of tile and cover the flat surface & Let dry. Go to a tile store or online and buy a larger travertine or similar ceramic tan/brown soapdish big enough to cover it and thinset it in one fell swoop right over the hole and damage topically with a medium bet mortar also from creepshow its called prolite. Total cost maybe be $100 and youre elbow grease? im not a creepshow fan nor custom guy so much but its ez and 1 stop for you. otherewise tear the whole mess out ! this one is 8+ inches how bigs the hole?

Hire a plumber to properly insert an inflatable test plug and fill the pan. You will very quickly FIND cause. It is something you can do yourself but to be thourough the test plug should be inserted down the stand pipe Below the drains flange. If you do find a failed connection or otherwise leaking shower pan, the best way to avoid this in the future is with a flood tested shower pan. Many tilers even today skip and scough at Flood testing their own work in leu of hurrying....