+1 for Time Machine. It needs to be a very large drive, ideally 2 or 3 times larger than the total data she has. Make sure it's backing up both the internal iMac and the external data drive.
This is a partial solution, though, since it protects only against drive failure (it also can help find a deleted file). I'd recommend an offsite solution, too, to protect against fire, flood, burglary, etc. One of the online systems like Backblaze is just as easy to use as Time Machine, though the initial upload can take months. Or get another large drive and swap it with the Time Machine drive every week or so, making sure to keep one of them elsewhere (I keep my work drives at home, and my home drives at the studio).