> But the next day, for the first time ever, I started seeing ads for wiper blades on my feed. They disappeared after a few days, but it was spooky, as it was for an item that I have never ever discussed, or seen on my feed before, or since.

Could be something like this: Facebook knows from location data you were driving/riding together. Soon after your ride, your friend searches for wiper blades. Facebook then shows you wiper blade ads based on that knowledge.