Code requirements call for all shower valves to be meet anti-scald abilities...now is the time to replace that (and an inspector would require it).
You can buy a heat shield, but often just a large can with the ends cut out and then slit so you can fold it out will be sufficient to protect things. They also sell an aerosol foam that helps protect things while soldering. Just keep a spray bottle with some water in it and maybe a fire extinquisher available.
The harder thing may be draining the lines enough so you can actually solder things. You may have to shut the water off to the building if you don't have individual units, and then draining the lines is problematic if you can't open valves above you. Also, and this may be a big one, in many places, you cannot legally do any plumbing work in a multi-family unit unless you have a license. Singel family dwelling is another matter, but you're dealing with multiple families health here...the fines and consequences can be huge.
One code-approved, solderless fitting is a Sharkbite. HD sells them, and you should be able to find them elsewhere as well. If you follow the installation instructions and don't mess up the o-rings, they work well and should last. I wouldn't use them for everything, but you could solder the bits to a new valve, then use a coupler to mate the assembly to the old pipe stubs...but then, I'd probably just solder things. The licensed plumber issue on multi-familiy dwelling may or may not apply where she lives.
For help in tiling the alcove when done, check out www.johnbridge.com.