I hate to tell you, but St. Johns Wort is used for actual medication, your friend wasn’t prescribed some ‘herbal tea’ the plant is known to be an incredibly low risk treatment for anxiety and light forms of depression, side effects are very rare, but due to the way it might interact with other types of medication it can only be sold through a pharmacy.

This is in no means an attempt to undermine your experience, but what most likely happened is that your friends doctor was at fault, he should have been aware of any other health risks, or at the very least should have asked before prescribing the medication, and should have stopped the treatment the second symptoms for stomach related illnesses started appearing.