I'm late to this thread (sorry!), but after scanning 20 pages or so I can't seem to find any real viable theories, I guess there isn't much to go off of.
That said, I did watch the video, and Gabby to me seems a little unstable. She admits she suffers from OCD, and also that she hit the guy. I realize those are small things, but some people really do lose control a bit when in a fight and start hitting and screaming and stuff. He seems pretty calm in the face of police being around.
I had a girlfriend when I was 27 years old, and she was really unstable. I came to a point where I wanted to end up, and was making a plan to do so (you have to be careful when ending a relationship with someone unstable). But one night we were at a party having a bunch of drinks and I just hit my breaking point. After we left the party, I just decided to get it over with and told her I was really done and I didn't want it any more. Of course now that I'm 44, I wished I would have handled that with a bit more empathy, but I was just tired of all the **** and wanted it to be done. After I broke up with her she promptly threw herself into a river and was swept downstream. I looked for her for about 5 minutes, but realized it was a scary situation and she could be really hurt.
I flagged down a passing police car and told them the story, after which they promptly put me in the back of the car and we drove around looking for her. I immediately sensed I was a person of interest, and now possibly in some trouble because they assumed I had tossed her into the river or something. Thankfully for me (and her) we saw her come out of the river downstream after looking for 20-30 minutes. The cops approached her to find out what really happened (which is exactly what I said), and basically had a meltdown with the cops and started punching and screaming at them. In the end I was released that night since my story checked out, and she was involuntarily committed to the psych ward that night. Of course, we stayed broken up.
I'm not saying any of that happened, and he's been extremely shady so far, especially with not talking to the police and disappearing. But maybe he just had enough of her and decided to leave, and in the midst of a breakup, she had a breakdown and either threatened to hurt herself or ran away from him. In that case maybe what he's feeling now is immense guilt and shame that he didn't go looking for her (which is what I would have felt had my ex not come out of that river and I didn't flag down that cop). In this is this day and age when young people especially become so tied to an online persona, especially when it shows them happy and golucky and travelling that a breakup now is such a larger deal since it means the end of that life and being in the spotlight.
Of course, I know in these situations the guy probably did end up hurting her, but just want to present an alternate theory based on some of my own life experiences that almost got me into a load of legal trouble (i.e. if she hadn't stepped out of the water at some point and attacked the cops, I could have been in real trouble, even though I didn't do anything).