It amazes me how fast some landlords are in rationalising that every single bit of damage to their properties must somehow be the responsibility of the tenant.
No... landlords are responsible for their own properties and tenants are responsible for their own possessions.
The landlord is contractually obligated to provide a functional house. If the house actually was functional and fit for its purpose, then it wouldn't have leaked water everywhere and damaged the tenant's furniture.
No... until any tap fails, it is functional and fit for its purpose. Once it springs a leak, it is no longer fit for purpose and the landlord needs to fix it. If the tenant's belongings are damaged they can claim on their insurance. If they don't have insurance, they shouldn't be able to hit up the landlord.
Of course the landlord didn't do anything wrong - but that's not why he is liable to the tenant. It's the plumber or builder's fault, but certainly the tenant is entitled to claim against the landlord at first instance, because that's who the tenant has a contract with.
If I was the landlord, I would pay out the tenant for the damage, and then be seeking that exact amount plus my own damages right from the plumber/builder/their insurer.
Good luck with that...