I don't wean my foals until 6 months. I think, the longer, the better. I've bought a foal weaned at 3 months and the difference in a lot of factors was not good...subtle, but had me wondering "what if"
Its a big step for the foal, not only nutrition-wise, but emotionally and growth-wise as well. At 6 months, they are old enough to breeze through all of these potential set-backs without a problem. You want to keep his growth moving along smoothly and uninterrupted. This is important to the growth of his bone and the bone plates that have still not finished growing and closed up.They are very suseptable to changes and will react if under stress. Not only will an interruption in nutrition disturb how the bones are growing, but add an upset youngster pacing a stall or running around the field calling for Mom and you have a double whammy on those bones. Letting her out with a friend so that she is not alone and is not frantic cause she has some company, plus old enough to not be traumatized by the separation. Old enough to eat all that she needs and weaning her is not going to make that big a difference. I'll go the full 6 months, though I know that not everyone does, but I believe that the whole thing is easier and better down the road if you give them the time to survive weaning with the least amount of risk to any kind of setback. She should be eating grain, but don't push the amount, complimentary amount and less is best. Overfeeding can cause the growth plates to get problems, like the inflamation of epiphisitis. You should always keep a couple of ribs showing and trust in Mother Nature to keep things balanced. By the third year, she will bulk out on 4 legs that are strong enough to hold the weight and a well-adjusted brain telling them what to do. Keep her wormed, vaccinated, your eyes open and ward off trauma and trust in Mother Nature and things should go smoothly. Best wishes!