Assaulting someone and leaving them lying on the ground until they die (whether the assailant looks at them or not) is very likely second degree murder. The push is a physical force which is known to be able to cause injury - which it did in your example (or else Gabby would simply have walked 10 minutes to the main campground or flagged down another camper). She was left there will all her things. So this assailant committed a felony (assault, DV, ending in death) and then took off with all the victim's possessions...
People don't go off with other friends/boyfriends without their ID, phone, and their basic possessions. They just don't. If they are locked out/thrown out on rocks in a campsite, they get up and seek help - unless they are not able. They do not just lie there waiting to be strangled by someone else.
Your defense left out that she died due to strangling. He cannot leave not knowing she's dead, IMO. If he says she was moving around after he strangled her, he is lying because she died due to a crushed windpipe and, likely, pressure to arteries.
He did not just push her. Another person did not just happen open her, sitting on open ground with no camping equipment, and just randomly strangle her.
The strangulation is what the defense has to explain away. No reasonable person would believe the Rando story.