What a buyer can do is have their lawyer negotiate a holdback from the sale proceeds to cover the fix, or in a serious case, argue the seller failed to deliver the home in the condition the offer required, which is a legal question for the lawyers, not something settled at the walkthrough.
Walking away entirely at that stage puts the buyer's own deposit at risk, since possession is normally treated as legally required once every condition in the offer has already been satisfied or waived, so refusing outright is rarely the buyer's best option even when the walkthrough goes badly.
Joel Dyck gets the seller's agent and both lawyers talking the moment a walkthrough problem shows up, so a bad walkthrough becomes a negotiated fix rather than a standoff. Get your hand-reviewed valuation from Joel Dyck.