Whether she can break the lease is going to depend on what she is allergic to and the current condition of your rental property. Many tenants complain that there is mold in the unit in order to get out of a lease. Until I have more information regarding the nature of the allergy and the conditions of the rental unit, I cannot answer your question. She is required to give you notice of any defect and a reasonable opportunity to cure before breaking the lease. It doesn't sound like she has done that. You should probably contact a local landlord/tenant attorney and discuss the specific facts of your case with him/her.
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It depends on what is is alergic to. If there is mold in the home it may be a good reason to vacate and you may have the posibility of a tort suit. If she is alergic to dust it may not be a good reason to vacate. The difference is that you have the duty to provide a safe home to your tenant. having mold in the home is not safe and is a violation of Florida Health code. Legall she is supose to give you a 7 day notice to cure and if you do not cure the problem she can legally move out.
A simple doctors note is not sufficient grounds to break a lease. Depending on what is causing the alleged reaction it may or may not be in your interests to let her go without a fight--you really need the tenant to specify what is causing the reaction and you should consider contacting a landlord tenant attorney in your area--that attorney will want more details, including eventually the tenants medical history in order to determine if this is a trumped up matter b/c tenant has found a better place or if you have some habitability issues with your property.
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