Separate the residents immediately and document what was done and when, including any change in room, unit or supervision.
Assess and treat the injured resident, notify the physician and the family, and report to the state if reportable. Resident-on-resident injury is reportable in many jurisdictions and operators sometimes assume it is not because no staff member was involved.
Preserve both clinical records, both behavioural histories and the full aggression incident log, along with video of the common area.
Interview witnesses promptly, with counsel involved, and note that some witnesses will be residents whose accounts are both important and contested.
Convene the care plan review for the aggressor and document the outcome, whether that is a psychiatric evaluation, a medication review, increased supervision, or the beginning of a transfer process.
Notice the carrier, and specifically ask the coverage question in writing: does this claim attach to the abuse endorsement, the professional liability grant, or neither. Better to have that answered at notice than at mediation.