It depends whether in the context of this job and its staffing needs/workflow, not doing mandatory overtime is reasonable or not. Under the ADA, an employer must only make "reasonable" accommodations to a disability--that is, ones that are not too disruptive to the employer. If the job has always involved mandatory overtime, so that is a normal part of it, and if there is no other or insufficient staff who could do it for your fiance, then the employer can insist she does it--the long hours are a natural or normal part of the job, so it would be unreasonable to not have an employee put in the hours. On the other hand, if this is occasonal or sporadic, so it's not a core part of the job, or if there are other employees who could handle the overtime instead, it would be reasonable to exempt your fiance from it and to not require it.