Gave IPhone 4 to Husband.

I'm helping a friend. She decided to upgrade her IPhone 4 to and IPhone 5c. She gave her old phone to her husband. Away they went to the Source to talk about plans and a new phone etc. they took the old phone with them. She got her new phone with the same phone number. i guess they put a new SIM card in the old phone with a new phone number. But here's the thing. The old phone still has all,her apps, it's still receiving her texts and emails. as well as on her new phone. So its like she now has two identical phones.

should I reset the old phone and remove it from her devices? If so, what will happen to the phone number the Source created on the IPhone 4? Will it be gone? How do we make the old phone become the husbands.