Steve - Agreed as suggested above to remove all the mushy black rotted parts. Use a sharp knife and clean it between cuts. If you are very lucky, part of the stem and upper leaves will still be okay.

This is definitely something that will take quite a bit of time to grow back, if at all. I'd recommend finding a suitable replacement plant in the meantime! Hopefully she'll see your efforts to save it and you can grow some new ones together.

I do like your large red pot, but in this case it was just too large for your succulent. Perhaps plan on adding multiple plants to fill the pot better? That will help.

But most of all, see if you can go for a much less water-retentive soil mix. The easiest solution is adding perhaps 50% perlite (sift out the dust/small bits with a cheap kitchen sieve and don't breathe it in).

A gritty mix (something you can make yourself) is a very good option and does involve a bit more effort to assemble the components. However, it is proven

There are many experienced California growers here in the forums and they likely can give a much better recommendation on soil mix ratios that worked for them.

I'm in Canada, but my rooftop is a lot like parts of California in the summer! And my plants growing indoors are under lights and have temperatures a bit like Cali too (sadly not my whole house is like that).

Anyway, good luck with this one and if you do get a replacement(s), feel free to ask more questions!