Note that "iCloud" has a very specific meaning. The term "iCloud" is not the same as Apple ID/Account, but you may be using it that way so I am not clear what it is exactly you wish to do here.
If your wife is part of a Family Sharing group that has a family level subscription to iCloud+, if she leaves the group and uses more than the 5 GB each Apple ID gets for free, she will want to take out an individual level subscription prior to leaving the group. She does not need to transfer anything if she has her own account, she just needs to make sure she takes out an individual level subscription before leaving the group. Have her read this support article for instructions on how to do this --> Upgrade to iCloud+ - Apple Support