Lost contacts after updating on another device

iCloud mixup?

if I had to guess what happened, it sounds like your mom has iCloud setup with her Apple ID on her devices. Then when your dad got set up with her ID/iCloud, his contacts replaced hers on the iCloud account, which would mean your mom's contacts would be removed from your mom's devices

when your dad's contacts were added to the phone, were your mom's contacts also there at first, and then manually deleted?

if that's how it happened, there may not be many options to easily recover them

are your mom's contacts anywhere else, like in an address book or contacts app on a computer? you could also check for backups of her iPhone and iPad in iTunes on a computer, or in iCloud