I wouldn't say anything up front unless I knew I would be in the bathroom a lot and if you know you are going to be on the toilet from 9 to 11am and you know that you need to be there at that time, then yes, I would suppose you'd need to tell them why you wont be at your desk for hours.
I'm working on the assumption she's in a pretty active flare. I interviewed for a job when I was first diagnosed and was still very ill. I'm looked ill - thin with pred moon face. I didn't spend hours in the bathroom, but I was in and out alot. I had to hit the bathroom as soon as I arrived at the building after a 20 minute commute. I figured it would be obvious something was wrong. For me, it's less stressful to have the cards on the table. I had a great boss who was very supportive.
By the time I interviewed for my current job, four years ago, I was in remission (and had been stable for a while) and didn't mention it during the interview process. It's a judgement call, depending on your current state of health.
KarKar, an agency dealing with the disabled sounds like good place to consider. Anything that's not fast-paced, cut-throat and competitive.