The Android Memo app. I would have thought, data is retrievable unless overwritten?
It depends on how whatever app this is (as stated above, there is no standard Android Memo app) stored the data.
If the memos are stored on a removable SD card this is simple: remove the card from the phone, stick it in a card reader, stick that in a computer and run any old file recovery software you like.
Otherwise, I'm afraid you are almost certainly out of luck. A non-privileged app (i.e. any app the user can install on an unrooted phone) is not likely to have the level of filesystem access needed for file recovery. And you can't just plug it into a computer and run file recovery software like you would on a memory stick because file access is granted using Microsoft's Media Transfer Protocol rather than mounting the storage as USB Mass Storage, and you can't just run file recovery over that. You will find apps advertised on the web that claim they can do this, but verifiable reports of success (i.e. independent of the company selling the software) are hard to find.
Worst case is if the app stores its data in the "private" storage area, where apps' own settings etc are stored. Because in that case you need root access even to see where the data are. And on most phones the process of rooting will involve a factory reset, and since the data are encrypted and the encryption key is toasted by a reset that really is game over.
I wouldn't rule out a serious forensic data recovery company being able to do this, but that would cost, probably more than the phone. DIY though, it's unlikely.