I presume you are using a low to the floor bed and a mattress on the floor for the person to roll out on?
I don’t know how mobile this person is.
It is possible to get cot sides for low to the floor beds that provide some sensory feedback so the person knows that they have reached the edge of the bed. However, if using even soft padded cot sides, you will need to operate the restraint minimisation protocols for your organisation and they may be an injury hazard if the person has the ability to climb over them even at floor level.
You could try an invisibeam alarm that will alert the nurses to re=position the person back in the middle of the bed.