What seems to have happened was a pair of problems. The first was a large clog in the common drains. That serves not only our two showers, but those of the two floors above us. Responsibility for that goes to the condo association. However, a portion of the drain in Cheryl's shower was also shot, and that's a "local" problem, now repaired. The backup in the drains was causing water to back up into Cheryl's shower. The reason my shower leaked into hers was because hers is "downhill" from mine. Anyway, fingers crossed that the drains are all working again.
There are still things to be done before we can put the apartment back together again. A workman needs to come and patch the hole he cut in the drywall to access the pipes. And Cheryl's shower really needs re-grouting, although the plumber says it's not actually leaking. Still things are looking better than they were this morning.