There's nothing, in particular, of health concern about corn syrup or high-fructose corn syrup besides it being sugary. It is used in everything because corn is incredibly subsidized by the US government and so the syrup created from it is very very cheap. The amount of sugar in general in processed foods is a problem if eaten to excess, but whether that sugar comes from cane, corn, or any other food safe plant, it makes little difference for overall health risks.

If this person is having digestive issues/discomfort from it, I would suggest that one of 3 things is happening. 1) it's psychsomatic because they assume it's worse for them. 2) They have an allergy to something in the drink. Maybe the corn, maybe something else. 3) They have fructose intolerance from fructose malabsortion in the small intestine. If the fructose isn't all absorbed before making it to your large intestine, it can ferment some from your gut bacteria and cause bloating and gas. This is uncomfortable, but not dangerous to your health.

Fructose is not chemically dangerous and it is naturally found in many foods (fruits, honey, vegetables, ... corn, etc.). The only difference between the glucose in cane sugar and typical corn syrup and the fructose in High Fructose Corn Syrup is that fructose is a larger more complex molecule, tastes sweeter, and might absorb slower in your digestive tract (which as arguably better for your blood sugar and calorie intake). There is good reason to be concerned about HFCS in processed foods, but the concern should just be that it is over used due to it being a cheap way to make foods taste good, and we have too much sugar in our diets overall.