I'd love to see tambo try and stand up in court and say it's the landlords fault for renting to a tenant that sleeps!!
The tenant should have had insurance to cover this. the landlord insures the property to cover against faults in the parts he owns (i.e. burst water pipes) and the structure of the building, the tenant insures what they own or may be responsible for (i.e. if an electric fire owned by the tenant caused a fire or, as in this case, the tenant causes damage by leaving a tap on).
As it stands the landlord should be bale to claim from his insurance, but his insurance will then try and reclaim the money back from your MIL because the damage was caused by her (there is no getting away from this fact)