sounds like they’re just eating less, which… yeah that do be how to lose weight.
he’s probably not having much fun on the toilet though, unless he’s also taking fibre supplements.

but yeah it’s not a good idea, you should be really really really careful about diets that have you eating the same thing often. Diversity in your diet is very important to make sure you get all the nutrients you need and in sensible amounts, as well as avoiding eating too much of any one thing which might cause problems.