I've bought some betta's from lf's and they are still OK 6 months down the line.
Maybe they are using RO water, and when I have bought fish from shops that use RO systems I gradually introduce the fish to my tank water over a weeks period but that is only cause I have a spare tank that I can do that with.
If you don't have a spare one then what I would do is use a container to float the fish in the tank using the original water he came in and do a gradual replacement of his water with your tank water, but I wouldn't rush it I'd take a few days so that his system isn't shocked.