Can we turn a phone in early if person is leaving our family plan?

What is to turn in?

She or you pays off the phone then she can take the phone with her (BYOD) to her new account or you / she can sell the phone.

 Once you remove the phone from the account, the remaining balance becomes due. 
The phone would need to be paid off to transfer to another service or account.

The short answer is you cannot just turn in the phone and be relieved of the remaining 11 payments. You have to pay it off