He could have killed her.
However she also may be in hiding to screw him over.
Actually, he didn’t have to leave her body in Utah. He could have kept her body in the back of the van and disposed of it in any state on his return to Florida. I guess it will depend on how much tech information the authorities will be able to discover and what route he took on his return.
The family received the last text which indicated there wouldn’t be any cell service in Yosemite, which was never a travel plan and hundreds of miles in the opposite direction of where they were supposed to go. The family doesn’t believe the text was from her.