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!