Disabled adult tenant not on lease

[Joe H.]

I'm not looking for legal advice from this forum and will escalate this to my attorney if necessary.

I'm currently under contract to purchase a fourplex and during my due diligence I have determined that one of the apartments has an additional adult living there who is not on the lease. This was brought to the attention of the current property manager who has indicated that the person in question is disabled and "protected under the Americans with Disabilities Act." 

I'm not an expert on the ADA or Fair Housing laws, but is there any reason that an adult with a disability would be legally protected from being on the lease?