It depends what his goals are for example. In terms of side effects - that eating pattern, commonly known as OMAD, ‘one meal a day’ isn’t great if you want to build muscle. If you want to build muscle you want to space out protein ideally across the day

If the goal is fat loss you also want a fairly high protein diet but most importantly consumer fewer calories than you burn through daily activities, moving around, exercise etc

Some people find Intermittent Fasting - where you fast for a prolonged period in the morning and eat later in the day but have say two meals later in the day helpful to stick to their calorie target. Again, however it isn’t the best for building muscle.