There are no guarantees that either swan will accept each other which is only one issue.

The major issue is that in Canada, your friend may need a federal permit to maintain a swan or keep a breeding pair. With all of the problems generated by having to notify the officials that she has a swan, it may be better that she enjoys the swan she presently maintains. The ducks will keep the swan happy with companionship and everyone will be better off without creating many problems with getting another swan, especially a male which produces cygnets. Then, she would be considered a breeder with many more issues to address. The Regal Swan