This is a very common and completely normal phenomenon for a kid his age (and older).
The 3 meal/day thing is completely against most of what we'd naturally do. Normally, we'd be more of grazers, so if he's snacking throughout the day on healthy things like fruits/veggies, nuts, cheese, etc., he's probably getting enough nutrients.
If she's really concerned, she should call the pediatrician and see if she needs to supplement what he is eating. I would strongly advise against using a multivitamin until talking to them as doses can be really tricky at that age. our pediatrician has both of our kids (3.5 and 23 months) on 1/2 Flintstone vitamin/day.
The American Academy of Pediatrics says the following things about picky eaters:
http://www.healthychildren.org/English/ages-stages/toddle...