If they had a fight and parted during the trip, he might not know what happened to her. It’s certainly easy to think the worst though.
No matter how innocent one is, getting a lawyer is the smart thing to do.
There was a case in Atlanta over 30 years ago where the boyfriend and girlfriend had broke up and she went someplace and her car broke down and was killed. The boyfriend was considered the prime suspect for a long time until a girl ratted out another guy with rape and robbery being the motive.