My daughter with the frequent nausea and GI problems has found that the mastic gum you suggested helps. It hasn't stopped it, but cuts it down considerably. She took mastic 2-3 times/day for a couple months and then stopped, since resins are hard on kidneys.

However, her symptoms have crept back. So she is taking mastic once per day and finds it helps enough. I would call this a maintenance dose, I guess.

Will this still be hard on her kidneys - one capsule per day? She is 12 (about 115 lb). Can she continue this indefinitely or until her issues resolve?

She should be fine with the mastic long term.  Mastic will kill H. pylori in very small concentrations, so she does not have to take large enough doses to be an issue as with some other resins.

I would combine it with a little licorice root though to increase effectiveness.