I work with a man that’s 37 has top teeth rotted out says its from too much fluoride where he use to live. Can too much rot teeth? New to me

Fluoride. Fluoride in municipal water supplies is regulated to therapeutic amount of <1 part/million. That helps protect teeth. Well water in some areas, particularly in the midwest, sometimes has >3 parts/million, which can cause brown mottling of teeth. If teeth have large untreated cavities, despite the fluoride, restorative/replacement dental care is indicated. Your coworker is overdue for dental care.
Fluorosis. Excess fluoride in the drinking water while the teeth are forming can cause mottling of the enamel (dark brown staining) that can appear like tooth decay. If his teeth are truly severely decayed, he should have them restored.