Shirley Last Name Withheld wants to help her boss, who constantly loses his phone, without having her data get intermingled with his. She notes:
Although I sign into iTunes using his Apple ID—say firstname.lastname@example.org—and sync his phone with iTunes, all my data (photos, contacts, etc.) goes into his phone as well. How can I separate his stuff from mine?
If he’s switched entirely to iCloud syncing, iCloud backups, and iCloud Photo Library, you could avoid using iTunes altogether, but it sounds like that might not be in the cards. If you can make that happen, you’ll probably need to make sure he has 50GB or 200GB of paid iCloud storage as well.
Read more here:: When iTunes accounts intermingle: How to separate the data