Welcome to the board.
The dog is probably reacting to the changes in her life, rather than bad care. That is pretty common. Your daughter probably is also having a hard time-divorce is tough on everyone, especially those who did not have anything to do with the situation.
I agree with establishing 50/50 time with both the dog and your daughter. Many studies have shown that amicable, equitable parenting schedules work out best for the children, and I am sure dogs are no different.
I would also suggest working with your ex on keeping commands and rules consistant, it would help Marshmallow adjust to changing homes.
Again, welcome to the board.