She will do just fine on her own. Swans may or may not take another mate/swan pal if the first one dies. She certainly will not be accepted by the male/female pair at your friends as they will chase her during mating/nesting season.

As long as there are other waterfowl (ducks, geese, etc.) in the pond, she will have enough companionship. If there are no such waterfowl, you might introduce a young goose or young ducks. In any case, you do not want to cause her stress by removing her from the only home she has known or placing her in a non-familiar habitat with other swans that may chase her.

How did the daughter die? Was it illness, poison, injury or predation? This would certainly dissuade trying to get another swan until you know for sure how the daughter died. The last thing you would want is to introduce another swan and have the mother and another swan meet the same demise. Also, an introduction would involve an enormous amount of labor to build an introduction pen, etc., and you may need a permit to have a breeding pair.

For these reasons, it would be better to let her live her life alone and in the same habitat she is familiar. The Regal Swan