What can be done for a Betta who has cholrine poisoning?

Unfortunately, this is just a tragic accident and there isn't any quick fix to save your betta. All you can do is get the betta into livable water as soon as possible and then work to slowly bring it back to it's ideal range.

Also, continue to monitor him daily for signs of disease. Sometimes bettas are able to survive traumatic events but will later come down with diseases as a result of a weakened immune system.

If your Betta is still in chlorinated water, the best thing you can do is add your regular water conditioner to the tank as soon as possible to avoid further toxicity.

If your Betta appears to be struggling to breathe, which can sometimes happen when exposed to poisons like chlorine, ammonia, etc; you may find adding an air stone to tank can ease the burden somewhat. He will continue to use his labyrinth organ to get atmospheric oxygen but may be able to retrieve more dissolved oxygen from the water if there is an air stone to supplement.

Most people have tap water that is treated with chlorine and/or chloramines to prevent pathogens from infecting us. While it's safe for us to drink it can be deadly to our fish.