My friend is losing weight by only eating once a day . that can't be healthy right ?

Well....: Arguments on both sides. As long as nutrition is adequate short periods of restrictive dieting is just as safe as a fast. Prolonged periods with severely restricted caloric intake ( wt loss greater than 1-2 lbs/wk) put one at risk of nutritional and electrolyte deficiency.

One a day meals: can make blood sugar (and mood) erratic. Better off to "graze" i.e., small amounts of food every few hours, especially protein. Peace and good health.