Haha, actually someone online did post once that they saw their betta choking. Worried, the person reached in and tapped the fish very gently on the back and the pellet he was choking on shot out.
That aside, I'm not quite sure what can be done to help a choking fish. Probably the best thing is to prevent choking in the first place by making sure their food is small enough or soft enough for them to eat. Bettas are gluttons, they don't chew their food but slurp it up whole. Plus, bear in mind that pellets and freeze-dried foods do expand a little in the water.