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A friend of mine told me an anecdote about his mother, who drank too much caffeine, to the point she became hypotensive and would pass out. Because caffeine acts as a stimulant, I'm assuming the decreased blood pressure was some kind of tolerance effect.
How exactly would caffeine have disrupted her circulatory regulation and caused her normal pressure to decrease?
I can't speak to the causes of hypotension, but you are indeed correct, caffeine is a stimulant. As a stimulant, there is a well documented acute period of hypertension that lasts for up to 4 hours. Interestingly, there is no causal link established between caffiene consumption and chronic hypertension leading to cardiovascular disease (see here and here).
Having said that, caffeine is also a diuretic, which could cause someone who is already fairly dehydrated to exacerbate that state. It's possible to faint from a result of severe dehydration but I think it would have to be pretty severe.
It's possible someone is prone to fainting for non-medical, purely physiological reasons, such as standing for long periods, standing up too quickly, hypoglycemia, or some other predisposition.
Caffeine's mechanism of action,, Hindawi journal (https://www.hindawi.com/journals/ijvm/2010/834060/)
Direct effects of caffeine: On vascular smooth muscle: Caffeine inhibits phosphodiesterase enzyme , which is responsible for degradation of cAMP to 5'AMP , so concentration of cAMP increases which activates protein kinase A which phosphorylates myosin light chain kinase (MLCK) . MLCK, on activation by calcium- calmodulin complex, phosphorylates light chain of myosin which then binds to actin and causes smooth muscle contraction. But MLCK in phosphorylated state is insensitive to calcium and hence can't be activated which results in relaxation of smooth muscles . So caffeine increases MLCK concentration in phosphorylated state which is insensitive to calcium hence cause vasodilation
Indirect effects: On endothelium: Caffeine inhibits phosphodiesterase, required for degradation of cGMP to 5'GMP. It increases nitric oxide (NO) production in endothelium by activating nitric oxide synthase via increased cGMP . NO then diffuses to vascular smooth muscles where it causes relaxation which results in vasodilation. Hence caffeine causes vasodilation by both direct and indirect effects.
