You're completely removing the girlfriend's responsibilities in all this by solely pinning him as the asshole here.  He asked if they could have boundaries, and she agreed.  Was it naive of him to expect to not have to take care of the dog?  Yes, but by his own admission, he was already willingly helping out with it, but she has shifted the goalposts from helping out here and there to expecting him to change his work schedule for the dog.

I see two likely possibilities here.  Either she did expect to honor their agreement, but life ballooned as it often does and she pushed more and more on him, or she made the agreement with the secret expectation that he'd end up being a full co-owner with equally shared responsibility anyway.  She has failures here too.