Update:
The stairs has neither a handrail nor a railing on the open side. It's a cantilever staircase flush against a wall, with a brass handrail on the wall side. The open side has no barrier of any kind.
Local code officer says "new work must be to code", which does not clarify whether or not an existing item that an inspection indicates is a 'safety concern' is a mandatory repair.
Considering tenants of all ages, health, and states of consciousness, and for peace of mind, we are going to cancel $3k worth of bookings and install a cable rail system.
Thanks everyone!