Two Phones Backed up to iCloud with the Same Same Apple ID

I am helping my 82 year old mother back her data up to iCloud from her MacBook, her iPhone, and her husband's iPhone. Before I started the iCloud backup, she recently went to Verizon and got a new iPhone. She then handed her phone down to her husband. I had his old iPhone set up with his own Apple ID, but they left her Apple ID signed into her old phone instead of signing in with the ID I created for him. They did not have any problems, until I backed both phones up to iCloud. Everything is backing up fine, but she really was not fond of the fact that he can access her pictures. Now I am being told he is getting her text messages also. I know I need to get him signed in with his own Apple ID, but my question would be what happens to data on his phone when I sign out with her Apple ID and sign in with one I create for him? Any advice on how to proceede would be appreciated. Thank you.