What happens to teeth if someone drink hot tea and cold water together?

Helps with staining : Some people do drink water after tea to help with staining.

Thermal cycling: I will assume that your intent was to inquire about alternating between hot tea and cold water, as simultaneous consumption of both would require mind-boggling oral gymnastics. However, this scenario puts the teeth through "thermal cycling"-- rapid change between heat and cold. This is stressful for the teeth, and may induce pulpitis if the temperature change is sufficiently severe and frequent.

Nothing special: Unless you have inflammation in the nerve or a cavity it shouldn't bother you. Maybe short lived sensitivity. If the sensation lingers more then usual get checked for a cavity or possible root canal.