The landlady is at fault here.
In all aspects of law it is the landlord/owners responsibility to ensure that tenants and visitors to homes and shops are kept safe.
This relates to structural issues and gas, electric etc.
If she blames you for the wet carpet and giving her overshoes she has accepted that she saw you cleaning the carpets. You also confirmed this with the tenant. Therefore she was made aware of the fact that there was a potential issue. She made this worse by making them wear the overshoes.
Also your contract was with the tenant NOT the landlady , so in effect the tenant is her first line of attack. You had left the building some time before having completed your job, its not like they tripped over your macine, hose etc..
Tell her to jog on if she complains.