I updated to a new iPhone and gave my Dad my old one. He only really uses it to make calls so I just used my Mum's Apple ID when I set it up, to save me making him a whole new account that he probably wouldn't use. The only problem is that then when I added all of his contacts into the phone, it wiped all the contacts from my Mum's iPhone and iPad and replaced them with his.
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
