I've almost got my MSc in Chemistry, so while I don't have particular experience with ponies that much I know plenty about acetone! Acetone is generally a very volatile liquid - in other words it evaporates quickly. It would depend a bit on if you're using mixed with other things or pure, but it shouldn't be damaging long term at all. As other commenters have mentioned, rinsing their ponies or just leaving them out for a little bit fixed most issues. If you're not seeing anything strange, it's likely all evaporated already and you've got nothing to worry about. You can give them a little wash with soap and water if you're not comfortable with it though