First you need to determine if your existing wheels use a cassette or a freewheel.
https://pedalchile.com/blog/freewheel-vs-cassette
That is a very entry level bike (cheap) so it might have used a freewheel instead of a cassette. A new 5 speed cassette is pretty cheap but the replacement wheelset is not and even a cheap set is more than the bike is worth. You are probably far better off junking the bike. Spend some time looking for a used bike that fits her instead.
If you are bound and determined to use the old bike you can have new 24" rims put on the existing wheels but buying the rims and having the wheels rebuilt by a bike shop will probably still cost you more than the bike is worth. Figure needing a set of spokes too if you replace the rims.