Is it allowed to leave a nursing home resident in bed all day, everyday?

Speak to MD about orders; be certain bed rest not ordered. And usually optimun is up with lift if necessary in chair just for the lungs, to try to prevent sores. They will likely plead staffing, or that the person is in danger in chair alone and they cannot stay. You don't mention age, condition, hospice or lack of it. All of these things are crucial as well as pain when/if up. Discuss with staffing. We cannot know the resident nor the general condition. Wish you the very best.

It all depends on what they can do physically.

It is literally impossible to answer such a question without knowing ANY details about the resident. You have no profile info set up either.