Well, actually, we postponed this solution attempt in the hope that EA will release a fix that will not result in wiping the locally stored progess. The game was not synched with the G+ account before the crashing occurred. And, unfortunately, 50 gems were purchased for that game just a few of days before the game was damaged. These gems (or rather the zen garden slots purchased with them) will be lost because, unlike other purchases like plants, will not be available to other games connected to the same Google Play account.
Copying and restoring the folders during a uninstall/install apparently only helps with not needing to redownload the files again. The game progress, however, is apparently reset during the procedure. I've tried it with my phone where I don't have much progress to lose. And indeed, this procedure resulted in having to play through the tutorial and basic levels first before being allowed into the game menu. Apparently uninstalling and reinstalling creates a new game or player ID that doesn't match with information in the saved files.
In case a bugfix is not released within the next few weeks, we will sync her phone to G+ and from there to the tablet since the phone game has already progressed quite a bit and she would not have to start playing from the start again.
Right now I'm still hoping for a solution based upon this part of a support reply from PopCap:
"If you continue to have difficulty after trying the steps above, we apologize for the frustration and thank you for your patience. If you would rather not risk the loss of progress, we are continuing to research the issue and aim to release a fix in a future update to the app."
The "steps above" are basically clearing the app data of the Google Play app and starting the game with airplane mode enabled.