Adopt it out to a more loving owner. She obviously doesn't care about it very much.

pound

You don't have to pay for the kennel. Most boarding places don't require payment until you pick the dog up. Check the dog in under her name and send her a text telling her where it's boarding. She'll have to pick it up and pay for it if she wants the dog back.

the bitch is using you

Take the dog to a kennel, and give them her name and contact information as the primary caretaker/owner. You can give them your name if they want it, but just fill everything out like you live at the same address as her and her phone number is the only one that will work. Most kennels don't make you pay up front, so just tell them she'll be picking up on whatever day she'll get back. Let her know where the dog is and when they're expecting her, and then cut all contact. That's what she gets for having bad breakup timing. If she complains, remind her that she's lucky you're not spiteful enough try to keep her dog.

Let it run loose in her apartment/ house, let it destroy shit.

Or just get one of the bowls that feeds the dogs on its own and and a water bowl that self serves more water and leave so it can make a mess

Give it a big bowl of food take a shit in the floor piss on the walls say adios sucka

Take it to the kennel and give her the bill. Or take care of it until she comes back and charge her for it.

Damn... Do what you think is better for the dog, not for her.