It is becoming a common trend mostly amongst middle aged woman to drink warm water with lemon juice for weight loss purpose, I don't know much about effect om weight loss but I know the effect it has on teeth.
As a dental student I have learned your mouth has an optimal ph level and when sugar and something sour with ph under 5.5 cause demineralization of teeth leading to the formation of carries/cavities. Cause lemon is so sour it has strong influence on the enamel of teeth causing structure to change. This is depicted by the Stephen curve when pH in mouth bellow 5.5 tooth demineralize causeing decay.
My tip drink water or eat cheese after drinking water this will cause ph to rise and work against demineralization ,and if possible whould be best to brush teeth.
Protect your teeth cause , dentistry is not expensive neglect is!