Based on my experience the when my weekday Niagara local train failed to take me to St. Catharines a few years ago, GO won't refund the tickets, because the delay was CN's fault.
My experience:
The Maple Leaf a half hour earlier had apparently reported unsafe track conditions on the track in St Catharines station, so our train had to take the other track, which doesn't have a platform. They gathered everyone (20 people) accessibility car, and the plan was to unload across the other track using the station's asphalt crossings which are there for that purpose. But for whatever reason, they decided that they couldn't do that and ended up running non-stop to Niagara Falls. They said they would run a shuttle bus back to St. Catharines for us.
There was indeed a bus waiting in the parking lot of the train station, but upon exiting the parking lot (bottoming out on the steep ramp in the process) the bus just went to the bus terminal and started the regularly scheduled Route 12 trip to Burlington. So we stopped at Niagara College and were told that we'd need to transfer at St Catharines Fairview mall to another bus. The passengers basically revolted at this notion and demanded to be driven direct to St. Catharines Station. So the drivers arranged to instead swap buses at Fairview Mall, so the people heading to St. Catharines Station stayed on the bus, while people continuing on the 12 needed to get off switch to the other bus. There were a group of Route 12 customers upstairs who didn't speak English, but fortunately there was someone onboard who spoke Spanish and could explain to them what they needed to do.
I called GO afterwards to ask for a refund, and they told me that they wouldn't since the track conditions were CN's fault, not theirs. They only refunded the difference between the trip to Niagara which I had been forced to take, and the trip to St Catharines which I had been trying to take.