I apologize if these questions have been answered and I just missed the answers.
1. Was the boyfriend at home that night? Is it just assumed that he was texting her from their place or is that something we know for certain due to records of his phone pings, etc.?
I ask this because her going to get the pet could be something simple, like she found out he had decided to stay at a friend's house as well, and that nobody was home to put the dog out to do his business... or conversely, that the dog had been left out somewhere (chained? on a balcony? etc.) and the bf wasn't there to put him in. When your pet is your 'baby' you feel compelled to take care of their needs like people do with their human babies.
2. Do we know exactly where her vehicle was parked in the garage in relation to where she and the friend entered?
I ask this because it could explain why the friend did not stick around to watch her get in. If her vehicle was visible from where they were when they entered, in an almost-empty lot, it's not necessarily bad form on his part to not have walked her the rest of the way. If it seemed to be empty except for her car, with noplace for anyone to be hiding (other than in her car, which might seem unlikely if she had locked it)... I can see her waving the friend on.
3. Has the pubic been given an explanation for why the mystery friend was leaving the party at that time?
4. Do we know if she (along with mystery guy friend) was the last one to leave the friend's house / apartment? And if the friend was under the impression that she was going to return, and with the dog? She would have been re-arriving at the friend's house/apartment at approximately 6:00 a.m., dog in hand, had that been the plan, which seems rather odd. If you'd been up partying all night, even if you'd been fighting with the boyfriend, if you felt the need to go get the dog, might you not just say, "I'm just going to crash at my own place on the couch?"