I need some advice for my wife - Recruitment Business - public liability.

She rents premises on the first floor of a business complex. The unit she rents is on the first floor with another unit. Both units are owned by the same person. The park is sectional title. The stairs are common for both units and someone else rents out the other unit. She has installed a security gate at the bottom of the stairs with an intercom and buzzer.

She would like to know if somebody hurts themselves (staff or clients) on the stairs - who is liable, bearing in mind that once she put the gate up the stairs more than likely does not fall part of the complexes corporate body (she thinks) ?
She also needs to know who is responsible if someone injures themself in the unit she rents herself ?

Is she as the tenant liable or the landlord who owns the unit, but not the stairs, or the body corporate ?
She also needs to know if she is liable who is the best to contact for this insurance ? Would OUtsurance be tho one ?

Thanks