They are only responsible if they were at fault, which typically means that they did not take reasonable precaustions--i.e. did not salt as much as a reasonable property owner would salt. If that's the case--they didn't put on as much salt as the typical reasonable property owner--then they would be liable, though you'd have to sue them to get the money if they won't pay voluntarily. But if what they did was reasonable--that's what most owners would do--then even though your wife slipped, they would not be considered at fault or liable.