Usually, communities that are "55 and over" communities have strict regulations.  These can be found in the CC&Rs (Covenants, Conditions and Restrictions) of the community. If you have not already done so, you should obtain a copy and see if there are any restrictions regarding having the daughter live with the buyer.Without knowing further details, it may be that they cannot discriminate and deny housing to persons with mental disabilities.You should contact the US Department of Housing and Urban Development (HUD).  Below is the list of local offices:http://www.hud.gov/localoffices.cfmThere have been cases of such discrimination against the mentally disabled, which has been handled by HUD, and then referred to the Department of Justice.