I have refinished several bow with Helmsman Spar Urethane in a rattle can from Home Depot. It is a good finish. If you do it yourself, you want to remove all the old finish by sanding. What you do not want to do is sand on the fiberglass below the finish. If you do, you will reduce the draw weight of the bow, and there is a good chance you will mess up the tiller of the bow. Other than that, it is like refinishing anything. Just get all the old finish off, and then apply new finish in thin coats.
If the bowyer offered to do it for free, that is the route I would go for sure!
Bisch