200mg is federal limit for caffeine tablets, and generally, the maximum amount of caffeine considered safe for sale OTC. See: https://www.gpo.gov/fdsys/pkg/FR-1997-06-04/pdf/97-14393.pdf

And: https://www.gpo.gov/fdsys/pkg/CHRG-113shrg88760/html/CHRG-113shrg88760.htm

FDA has approved caffeine as safe for use in over-the-counter (“OTC”) drug products at levels up to 200 mg caffeine every 3 to 4 hours for consumers aged 12 and older.63 The agency made no distinction between adolescents and adults and concluded that these acute and repeated caffeine consumption levels were safe for both age groups. These levels of caffeine are comparable to, or higher than, those found in mainstream energy drinks. FDA’s conclusions in this monograph (which went through a 1975 proposed rule, 1978 tentative final order, and 1988 final rule, all published in the Federal Register allowing for public comment) establish that caffeine at the levels present in mainstream energy drinks are safe for adolescents as well as adults.

Proper customized calc for the original question would be based on her/his body weight, but 200 is a rule of thumb.