Here are a couple of suggestions. Ask the lady if she will take both swans. We have found that if the pairs can stay together through rehab, injured swans do much better.
As you stated, they have bonded and she is looking for him and no doubt he is probably concerned about her. So, try keeping them together during the rehabilitation process and even as he goes into surgery. If he should die, by taking her off the lake and he does not return, she may not be so stressed going around the lake looking for him.

Most female swans will re-mate if something happens to the original mate. But, this will have to be a slow introduction as you do not want the male to hurt her. He should be kept in a pen (half in the water and half on land with a feeder inside the pen) so that the female can have a chance to introduce herself to him and vice versa. This penning will also provide the male with a chance to acclimate to his surroundings.

After keeping the male in the pen for about a week, try to slowly introduce the two and if no signs of aggression are observed, things should be okay.

Also, try to get a male close to the female's age as swans can live up to 30 years in a captive setting with good veterinary medical care as well as good maintenance. Don't worry about getting another male until it warms-up. We hope that this information is of benefit to you. The Regal Swan