Your new roommate really has no ownership claim to the animal. Neither does she have standing (a legal connection to the harm) to make the claim. If the dog is in your possession, and you agree it belongs to the old girlfriend, your new roommate has an uphill battle on her hands.
I am not admitted to practice in Florida.
Your new roommate has no legal claim to the dog.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 385-8015 or via email at Eric@RothsteinLawNY.com. The above answer is for informational purposes only and not meant as legal advice.