Is it obligatory for a mentally disabled sister who has reached the age of responsibility to wear the full covering (ستر)?

If a woman is not of sound mind, religious obligations are lifted from her, due to the Prophet's (peace be upon him) saying: "The pen is lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane person until he regains his sanity." However, her guardian is obligated to protect and preserve her, and to keep her away from forbidden things, such as preventing her from appearing before non-mahram men without a hijab, and preventing her from being alone with a non-mahram man, and so on; this is to protect her and prevent corruption and fitna (temptation) through her.