Keeping a phone from a family plan

No idea how this works so looking for help. My girlfriends mom has a family plan with like 5 lines. Her and her mom got into an argument about her mom never visiting her grandchild. The mom got mad and told her to get her own plan, she then immediately disabled the line. We want to get her to her own contract but she has a pixel 7. We wanted to keep the pixel 7. How do we go about getting her off the plan without her mom as she isn't returning any messages.