Information on a standard platter hard drive can be recovered, however there are ways around that. If the platter becomes corroded, massively damaged, or destroyed then the data is not recoverable.
When you delete a file on a standard platter based hard drive, the O.S. loses the pointer/marker to the file. The file still exists though even though the O.S. will not be able to find it once you delete it. Someone who knows how to run a Data recovery program could recover it, even after you reformat the drive. The data can be overwritten, which would theoretically destroy the old data, but there are claims that this is NOT 100% effective.
A SSD is completely different since it uses a form of electronically charged Binary cells to store data. A SSD has a TRIM command that will go through and slowly erase the deleted files. Basically this Trim command takes the cells back from a 1 to a 0. You can also Secure Erase the SSD which will wipe the data off the drive. In fact, it's better to secure erase the SSD before reformatting it since this better organizes the cells. A Secure Erase pretty much flushes the cells by resetting all cells back to 0.
There are claims that a SSD can handle only a limited number of reads and writes, Yes that is true but overblown. A modern day SSD is capable of handling 1-2pb before dying. Older SSD's can handle 750tb write cycles before dying. Basically, a modern day SSD can handle a 1000 years of normal everyday use. Performing one or two Secure erases is nothing.
The only surefire way to delete the information on a standard platter based hard drive is to destroy the drive. If she has a platter based drive and you know how to use a recovery program, then you can dig up her old files. You can also have the drive sent to a recovery service, that will recover the files on her drive for a price.
People send their broken hard drives to a recovery service, because the parts that break in a Platter based drive are the mechanical parts like the Reader Arm or the Spindle. The Platter is still intact and the information can be recovered.
If she has a SSD, then you can kiss those files goodbye. Like I said about the TRIM command. When SSD's were introduced 9 years ago, the didn't have the Trim function. The drives would slowly decrease in performance because the drive would have to seek for places to write files and seek for where to read files. The only way to reorganize an old SSD was/is to Secure Erase it. The deleted files need to be erased in order for the SSD to organize it's self.