You could consider a range of sensors.
Abed pressure mat linked to a pager would be useful if you are most concerned about the client falling. It is discrete and will send a message to the linked pager as soon as the client gets out of bed.
Afloor pressure mat linked to a pager would send a message to the linked pager as soon as the client steps onto the mat. The floor pressure mat is more visible and may itself present a trip hazard. We would always recommend covering the floor pressure mat with a barrier mat.
A door sensor would alert you when the bedroom door opens, but is dependent on the bedroom door being shut at night. It also wouldn't alert you if the client is moving around their bedroom.
You could also consider a motion sensor which is supplied with a stand. This is portable and so could also monitor movement in other areas if needed. It can be positioned on the floor at the edge of the bed to pick up when the client gets out of bed. It could also be positioned outside the bedroom in a hallway or landing.