You have a problem that’s going to be difficult to deal with and the law is very unclear. If you’ve already provided one accommodation that you and the employer agreed would work for that employee, continue to make that person productive and useful to you. You’re not under any obligation to do everything that is possible to keep an employee even if they’ve got a disability. Whatever solution you have has to be reasonable for the employee and not produce an undue hardship on you. You’re going to have to engage in further discussion to see if there’s something that can make this work.