With all the cancelled flights, I can almost guarantee that AC will
add flights or send bigger aircrafts to pick up the stranded passengers. They just need time to get organized. Weather affects all airlines so I am sure flights are full on other airlines too... LGA is a busy airport and things can fall apart dramatically quickly. Your friend will find a seat tomorrow. Better safe than sorry... Doesn't seem like that YYZ or LGA
are having thunderstorms tomorrow so conditions should be better to fly. Good luck to your friend!