No, she's not really backwards.
At two years old, your horse is a growing boy--he NEEDS grain, especially
if he's in a dirt paddock (taking him to grass a few hours a day is really only a mental thing for him; it won't help him nutritionally). She is also right that SOME hay is just a filler--unless you're getting an alfalfa mix or some really good quality timothy, your horse is getting little to no nutrition. (look up the protein levels on grass hay--about 6%!!! Your boy needs around 12% overall to continue growing.)
(Also, I'm not saying the natural way is bad, just that most people cannot afford bales of hay a day for one horse and most horses cannot get enough nutrition out of grass because the area isn't big enough.)
ALSO take into consideration: if your boy was underfed, get him back to ideal weight VERY slowly. Growing horses who were underfed can have sudden growth spurts when given a lot of grain/hay/grass to 'plump' them up--which is BAD! Sudden growth spurts can have the bones growing faster then the muscles/tendons, so be very careful! (I'm not saying starve him, lol, just don't try for rapid weight gain!)
So, bottom line: Try to assess your feeding program. Your boy needs about 12-13% of protein total (seeing as he is still growing). Try to assess your hay honestly (there's charts online for that sort of thing), and figure out what percentage of protein he's getting there (also, try to get a feel of the WEIGHT of the hay, not the amount of flakes. Flakes vary in weights DRASTICALLY!) Then supplement with feed.
IMPORTANT: When you go to your feed bag, take off that little white nutritional information 'flag' at the bottom of the bag and take a look. SEE WHAT THE SERVING SIZE IS. TONS of people do not do this! If the bag says to feed one pound of feed for every 100 pounds of body weight, then that is what you need to feed to get the nutritional information that is on the bag. If it says he's getting 10% protein, x amount of fiber, and x amount of fat but you're only feeding him .5 pound of feed per 100 pounds of body weight, then he is NOT getting what the bag says!
Some feeds want you to feed way too much. Take for instance, Purina Mills Junior feed for Foals/Yearlings. I just purchased this feed for my weanling that got shipped in (it was what he was eating beforehand) and I am not impressed at ALL. On the back it gives you the 'pound-of-feed-per-100-pounds-of-body-weight' spiel, and it wants me to feed my baby 15 pounds of feed a day! WHAT? My grown horses in moderate to heavy work only get 8 pounds a day! I'm switching the baby over to platform feed, which is a lot more calorie dense then purinas feed.
Eating too much feed at one time is bad for one thing, usually--your pocket! Horse's stomachs can only hold about 4-6 pounds of feed (or 8 to 15 quarts? I think? Correct me if I'm wrong.), and if you give them more then that (which some uneducated people do to 'fatten them up'), all they do is poop it out! If the stomach is getting overloaded with feed it will, in most cases, dump it into the intestines. In minor cases the horse won't gain and might lose weight; in major cases, colic and founder are inevitable.
Hope all of this helped... if you need anything clarified, lemme know!