Notes app disappeared from iphone, how to get back?

This may be incredibly difficult. Scenario: my mom-in-law is 94 and relies heavily on her iphone 6 because they are connected with her hearing aids. She is out of state. My wife is currently with her and trying to rely on me for tech support because the NOTES app on her iphone appears to just be "gone"? She seems to find it in a folder called "FOLDER" but can't do anything with it. Trying to re-download NOTES from the app store puts the new NOTES app back into FOLDER.

I'm relaying this as succinctly as possible. I'm a tech geek but not with Apple stuff and I'm hoping this scenario might ring a bell with someone who can offer a possible solution?

Thanks a million!