I hope this message finds you well present circumstances excluded. I am a licensed attorney with over a decade of employment law experience. It is a pleasure to assist you today.
You can let her go without fear of legal liability. You need to be able to show through documentation that you have attempted to reasonably accommodate her relative to her illness. It appears that you have done that based on what you have shared with me.
She is likely eligible for disability based on her disease. The type of disability would be obtained through the Social Security Office. It is commonly referred to as SSDI. Once she is approved for that, in Illinois she may also apply for State Supplemental Payment (SSP) which helps offset costs of living.
She can apply for SSDI by meeting with the local social security office. About 30% of applicants in Illinois receive an award of disability. That said, due to the severity of her disease, her chances are much higher than the average.
I would first make sure you can document that you have attempted to reasonably accommodate her. Once you know you can do that through documentation and testimony, you can terminate. In the letter, I would state that "after many attempts to reasonably accommodate you and your employment, we have come to the conclusion that we cannot do so any further without unreasonable damage to the company." You can then give her perhaps a one month severance (one month of pay - though two weeks is reasonable as well and you don't have to give anything).
I would then add language that based on her condition, she may be able to receive SSDI and that you will work with her in attempting to get that accomplished if needed.
Tell her she has been a good employee, but can no longer being employed after good faith reasonable accommodation efforts have proved to be futile.
The letter should be short and sweet and to the point.
Let me know if you have any other questions or comments.
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Best wishes going forward!