If someone you know is very sensitive to acid foods, which foods would you recommend he or she avoid?

Most fruits, like oranges, lemons, limes, etc. contain citric Acid. So ... tell them to stay away from that stuff.

I believe milk contains lactic Acid, I. e. lactose/lactate. That could probably be avoided. (Although, the amount may not be high enough to be relevant)

There’s no telling how many other foods contain Acid.