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I'll add a bit of information. If the Garden was in that state before she moved in; then it is the Landlord that should have corrected problem(s) beforehand, before the tenant moved in.
If the Garden got in to that state after she had moved in; then it is not the Landlords responsibility but the tenants, because Garden areas are the tenants responsibility after they move in to look after it.
Also if the tenant fails to maintain the Garden and gets in to a state or needs repair work done to it i.e. slabbing stones need to be replaced; then the Landlord can charge the tenant for the work that is required to bring it back up to spec.
Anyway I'm just quoting my private housing Co-operatives (Easthall Park) rules in the handbook and also from what I remember from other things.