Anecdotally, my dad tried to lose weight with moderate exercise and light diet, and struggled to keep off 5 pounds; intermittent fasting (2 days a week of 1200

Anecdotally, my dad tried to lose weight with moderate exercise and light diet, and struggled to keep off 5 pounds; intermittent fasting (2 days a week of 1200 calories) dropped him something like 15 lbs in 40 days?