YouTube link.

Many people are surprised to learn that it is possible for a layman to repair a butterfly's broken wing. This video, prepared by the Live Monarch Foundation, begins with a demonstration of how to apply a cardboard splint to a fractured wing; all that one needs is a few household items and a modicum of dexterity.

The second part of the video shows how to attach a new wing when a major portion of the original one is missing; for this you would need to get one of the spare wings you keep around the house...

Some butterflies are truly ephemeral, with a lifespan measured in days; for those species a repair would not be practical. Monarchs however, can live for six months; a repaired wing may allow it to fly from the Upper Midwest to Mexico. Whether it's worth your time and effort is of course your decision.