My ex husband and I had our devices on the same Apple ID. Is it possible to back up his phone on my side and then restore the backup onto his device with his new Apple ID?
I know the purchases wouldn't be available but he wants to retain his photos and messages.
When restoring the backup, you will need the Apple ID, and password that was in use on the phone that was used to make the backup.
Messages won't be available if he is going to use a new ID.
If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos. Then make sure full resolution photos are on the device.
Any data that is in iCloud won't be part of the back up. When signing out of iCloud, if asked whether you want to retain data, choose yes. See Winston Churchill's response in the below discussion.
How to transfer all files from one iCloud account to another
Can I install a backup to a new Apple ID
