I would be curious as to what he is eating before he is going to class. Generally heartburn ca be triggered by poor digestion which means that maybe what he ate before did not digest prior to class. All I know is that my health completely changed for the better when I switched to the 80/10/10 diet. Which basically means 80% fruit, 10% veg, 10% fat. I know it might sound crazy and I had a hard time getting used to it at first, but now I can’t imagine living any other way. I never have any type of digestion issue anymore. My yoga practice has changed so much ever since then. Anyhow, hope all is well.