This issue has come up so many times. I suggest you do a search as you will surely find the answer.
But, alas, you must get the person to close their BIS account and release the PIN and IMEI. You cannot do that for him/her. They have to call the carrier and do it.
If it was on BES, the company needs to send a blank IT and/or remove all of the IT policies on the machine.
I suggest you wipe it and restore to factory settings as well.