Hi There,
There are two ways we could go about doing this. The first option would be to disable iCloud Photos on the iPad. When you do this a message will pop up saying "Do you want to keep a copy of this data on this iPad?" and you'll want to select "Download and Keep Originals". You will want to make sure that the iPad has enough storage space however to save all those photos/videos. You can check this in Settings > General > iPad Storage
The second option would be to go to www.icloud.com/photos and sign in with her iCloud Account and then download all those photos to the computer in a folder somewhere. Then, sign out of her account, sign in with your father-in-law's account and then upload the photos from there.