What you write depends on the situations. Was there abuse? Neglect? Is the person going to an AL or other level of higher care? Is it a financial issue?
As CM points out, more information is needed.
If there are legal issues, you'll have to wordsmith your response and be careful about any accusations you might want to make.
lbrantley, also check to see if you need to give 30 days notice that the patient would be leaving. Check the Lease to see what is required. There may be exceptions.
It is a simple matter of informing the IL that you will be leaving, I'm not sure why you feel you need to state any reasons. Read the contract's requirements and simply state "I'm writing to inform you that Mr/Ms so & so is leaving and we are hereby giving the required xxx days notice".
As CM points out, more information is needed.
If there are legal issues, you'll have to wordsmith your response and be careful about any accusations you might want to make.