never seen this before

[shipwreck]

ASA Class A State JO ball. Team is batting and has two runners on. I am PU and coach comes up to me and says that the girl that should be up to bat has left the field and he has no available subs. They take the out in her vacated spot. Third out. He then puts 8 players on the field for defense. After the inning is over a fan comes up to him and tells him he could have placed his DP on the field at the same time as his Flex. He hadn't realized that he could have done this. First of all this would be legal correct? Put nine on defense and take an out everytime the girl that lefts spot came up on offense. And if so, should I have alerted him that he could do this? This seems like coaching to me. Dave