Do they just seem darker like a filter was used on the photo, or is the thumbnail a black square?
Were they ever good pics or were they always dark, let's say when you previewed the pic after taking it?
If they just seem dark and they were never good, like in the preview after taking the pic, I would say your ISO setting has been adjusted from Auto, or your EV setting has been changed from the 0 position. I would first check your camera settings. Also if this is the case you may be able to save some of the pics with a photo editing app. Most will allow you to increase the brightness of pics and change contrasts to somewhat improve pics. They probably won't be perfect but better than little dark boxes.
Now if the pics were good and now they are not or if they appear as little black boxes, that could be more difficult to resolve. I don't know of any app that will mass edit pics, so for her to have affected multiple pics at one time seems kind of far fetched, but usually pics that have been edited and saved have an original pic still in there under a different name. So you may be looking at duplicates. Most of the apps I use rename the pic something similar to the original or use part of the original name so check for duplicates. If somehow the pic was saved with the original name and overwritten it will depend on what the edit was as to if the pic can be restored. Some edits can be reversed with opposite edits, and finding out if that is possible will be a matter of playing with it to see what you get.