Guardians may not admit the incapacitated person to a mental hospital. Chapter 190B § 5-309(f) 

A guardian must have specific authority from the court to sign an incapacitated person into a nursing facility, except in the case of “short term” admissions. Nursing facilities do not include rest homes, inpatient rehabilitation hospitals and assisted living facilities. The court must find that admission is in the person’s “best interest,” although the standard is not further defined.  Chapter 190B § 309(g). 

There is a different process for admissions to nursing facilities for fewer than 60 days. In that case, the guardian need not obtain court authority, but must notify the court of the admission. If the person under guardianship does not have counsel, counsel must be appointed. Presumably counsel will take action if the incapacitated person objects to the placement if the admission extends beyond 60 days.  Chapter 190B § 309 (g).