IPhone 4 question

I am wondering something. My mom has two iPhone 4's, and she wants to give one phone to my son, and keep her current phone. I am going to restore her unused phone, but wonder if this will affect her current phone. She has photos, email, and other items she wants to save. I am wondering if I restore the other iPhone 4 that is not in use, and has no service, if this will affect her phone that is in use.

Thank you