The one foal I have had, I weaned at 3 1/2 months on my vet's advice. The mare was getting very skinny and the baby was getting fat. She was eating all of Mom's hay, trying to steal her grain AND still nursing! The foal did fine and Mom's weight came up quickly.
A few years back, I also purchased a 10 week (yes 10 WEEK) old TB filly at an auction that the a-holes sold separately from the dam. The dam went to the killer buyers We started her on a junior type feed with extra supplements and added some powdered milk replacement meant for orphaned foals to her grain. I'm sorry I can't remember what it is called, but it is made to be mixed with water and bottle fed to orphaned foals. We just put a few scoops dry over her grain. She matured to 17 hands so I don't think she was stunted at all! She was very a very healthy horse.
My point is try not to stress too much about it. No, it is definitely not ideal and the owner is being a jerk, but you can still take steps to make sure your baby is healthy and happy. Good luck!