Transfer line and device Xfinity mobile to different Xfinity mobile account

I have been trying to do this for about 2 weeks now and I am extremely frustrated with Xfinity customer service.  I have spoken to countless Reps and spent hours on the phone with no solution. 

I want to transfer my Ex’s phone and device to her existing (newly created)  Xfinity mobile account. She has a line already on her account and wants to keep her number and device. Why on earth is something so straightforward so hard to accomplish???