That looks like steel.... As long as the tube that is just bent is not collapsed, A good bicycle mechanic or frame builder can try to cold set (bend without heat) that rear stay back straight. As long as the repair is done carefully, the repairs should not affect the paint/chrome and the brazing joints near the bent stay.
If the seatstay tube is collapsed or crushed, the tube can be replaced by a competent frame builder. Not cheap as you will also have to get areas of the bike that will have to be re-brazed to replace the tube, but it could be worth it if that frame is a top line/desirable one or if it just has strong sentimental value to the owner.
Only way to find out what real options you have is to show/discuss the frame with a competent frame builder.

Chombi