I don't get what your problem is, are you saying that your wife's phone

- cannot have any more apps installed
- cannot have any apps deinstalled
- runs out of memory, and then become unusable/useless?

Considering I've had several multi-MB pdfs (which will take 100s of MBs when rendered) open plus plenty of apps, I guess the system itself shouldn't use more than a few 10s of MB... didn't check yet.

On a side note, any good linux system will 'soon' have all RAM used, this is nothing to worry about (empty RAM costs as much current/power but is not useful).