Leave the betta in the tank. You will need to treat the whole tank no matter if you only see spots on one fish. The tank is "infected".
Will need to actually see a pic of the fish in question to make sure that it really is ICH.
I do not personally treat ICH with medication. I raise the temperature of the tank to 86F and add 1 tablespoon of salt per 5 gallons to the tank. Tank will need to be treated for no less than 2 weeks to make sure that all the parasites are gone. With the temperature being raised, I also suggest adding more aireation to the water(adding a bubbler, lowering the water level for HOB filters, ect.
It's also a very good idea to gravel vac thoroughly. Part of the ich lifecycle involves the parasites living and growing (reproducing here too? or is that in the water column?) in the substrate.
Thanks guys, I looked up natural ich treatment yesterday and am in the middle of raising the temp. Sadly I have a small power outage a the bettas will have to wait for a bit.
I have treated in this way (salt and higher temps) with bettas, cories, plecos, khuli loaches, otos, whiptail catfish, and various species/types of snails with no issues showing up with any of them.