Salt in aquarium tanks containing snails, in general, will cause them to produce a very cottony white slime excrete. This is a warning that "Theres something in the tank I don't like". This is, of course, if the salt is to concentrate in the tank. A small enough dose will still bother the snail, and over long periods of time may cause it to be completely inactive, but should not kill it in a short time period.
If you have a 1g bowl handy, you can simply house your snail there while treating your betta with salt over a week or two *Shrugs* Its simplistic, your snail definently wont mind.