If she does truly pose a risk to herself or to others and there is no fairly simple, cheap, easy way to prevent that--i.e. no "reasonable" accomodation--the employer does not have to let her on the premises and may suspend or terminate her employment. For example: say someone is a warehouse worker or machinery operator and is discovered to be narcoleptic or epileptic; those conditions, in those contexts, can put the worker's life, co-workers, or on-site visitors or customers in danger. The employer is not obligated legally to take those risks. So the answer depends on what you wife's problems are, how they can be moderated or controlled (if they can be), and what her job entails--that will determine if she is a risk, and whether the risk can be reasonably accomodated or not.