I build custom showers and also install tiled tub surrounds and here's my advice if you're wanting to "Shoot, Shovel, and Shut Up", prior to passing this problem on to some one else.
1: Stop using this tub/shower now...allow it to dry out. Do not use it
ever again prior to selling.
2: Find a matching tile and patch the hole. Try places like "Habitat for Humanity" as they may well have just what you need. Clay tile are not as easy to find these days...
3: Buy a grout saw and remove
all of the discolored grout; this is a definite tip off to an "honest" home inspector that the surround is most likely shot, and that it will require a full R&R in the immediate future.
4: Regrout the bottom rows and put a nice shiny bead of 100% silicone around the deck joint. Be very careful with the grout saw as the tiles are probably hanging by a thread around the bottom. A Dremel tool with the proper disc might be a better way to go. You only need to take out about 2mm of grout (1/16") so that the new grout will have a hole to fill.
5:"Run Forrest, run"...
I've torn more of these out than I care to even count and the water damage is always severe. There is a veritable miasma of molds and funguses living behind that mess...trust me on this. This is a can of worms you don't want to open...if you get behind the next tile with a putty knife? It will pop right off. So will the one beside it (and the one above it, and the one on the other side of that one, and so on, and so on). It's shot; and it needs to be ripped out and redone.
If you dry it out and "clean it up" then you'll probably "get away" with it.
Caveat Emptor (That's "buyer beware" in Latin, in case you needed to know).
The grout will start to discolor almost immediately so get it on the market asap once you've "cleaned it up".
'That's the best "advice" I can offer...