Hmmm....that's certainly a tough one and I certainly do not blame someone for not mentioning a chronic issue in an interview as (legal or not) it would certainly affect the hiring manager's decision-making process knowing that this individual may be out for long periods of time.
Instead of asking the employee when they think they may return (I find they rarely know themselves), it is probably best to get a doctor's certification stating when the medical professional believes the person in his/her care may return to work.
Is this a position that you could get a temp in to do so that the work is not as backlogged when/if the employee does return? This is something that seems like it could potentially go on indefinitely. If that is the case, then you must work closely with your legal department on ways to replace this individual since it may be placing undue hardship on the business.
Reasonable accommodation is just that; reasonable. If the business is going to suffer from having this position vacant, then action needs to be taken.
I feel for all involved, but at the end of the day, it is about running a business.