You're right. In order to recover, it is important to find and address the root causes.
Often there is a domino effect of body system dysfunction.
Gut health plays an important role in any chronic illness also.
Functional medicine practitioners approach any chronic illness by addressing a number of root causes such as hidden infections, gut health imbalances (imbalance of gut bacteria, leaky gut, lack of enough stomach acid, small intestinal bacterial overgrowth, etc), food sensitivities, heavy metals, poor diet and nutrient deficiencies, hormone imbalances, etc.
There is a lot of information online from good functional medicine practitioners about these topics.
I hope your friend is able to get help and recover.