I have to disagree with a previous poster, babies should never be given water. The water has no calories or nutrients and will replace something of value to him, whether formula or food. Infants have a low threshold for water intoxication and their electrolytes can become very imbalanced from only a little bit of water. Foods have adequate water in them to keep him hydrated.
I would recommend to your friend to feed less solids in any case. Babies should get the majority of their nutrition from formula or breastmilk at his age and really should only just be starting solids. Eating foods at this stage of development is more for learning texture and eating behaviors than nutrition. If he is a big boy he got that way from breastmilk or formula, not from solid foods. He will probably loose weight if too much formula is replaced with food.
On to your real question, I am guessing it is just a phase so I wouldn't worry too much about it. Babies/young kids typically eat when they are hungry, until they are full, and that's it. They will neither starve themselves nor overeat without good reasons. I would say give it at least 3-4 weeks before I was really concerned. Good luck!