As you correctly noted, Canada geese (and swans) do mate for life. Mated pairs not only raise and protect their young together, but also look out for one another over the course of their lives. One mate will stay by the other's side if injured or dying, even if the rest of the flock is moving on. They are extremely devoted to one another.

It is certainly possible that when one goose dies, the survivor will find a new partner. However, as with people, every goose is different. Whether or not a goose decides to pursue a new mate involves a number of factors, many of which we don't fully understand. Members of pair that have been together for many years are probably less likely to take a new mate than had the situation involved a younger pair -- but it is still up to the individual bird.

Canada geese are very social creatures, so the lone goose will always have a flock to be associated with when he or she chooses to socialize again. (Sometimes geese in mourning will stay by themselves.) It is possible that the goose will become a loner, but it is impossible to generalize. Again, it depends on the specific goose's "personality."