The buyer's options at that point are to requalify at the new rate, ask the lender for a short extension, which some lenders grant for a fee or a rate adjustment, or in some cases switch lenders if a better rate is available elsewhere before the deal closes through the parties' lawyers.
A financing condition in the offer is what protects a buyer here, not the rate lock itself. If the higher rate pushes the buyer outside what they can qualify for, a properly drafted financing condition allows them to walk away rather than close on terms they can no longer afford.
Because possession dates in Saskatoon can move for reasons outside anyone's control, building in a realistic timeline from offer to possession reduces how often a rate lock becomes the problem. Get your hand-reviewed valuation from Joel Dyck.