In answer to the biting issue (general) I would definitely have a go at the horse who has a go at me. In actual fact if someone came to me and said 'your horse bit me' the first thing I would say would be 'did you bite her back!!' I would be more upset if they didn't diciplin her. FYI my girl is a complete biatch who is known to have more than a nasty streak, she's a complete prima-dona.
Back to the running around scenario, if she is saying that your horse didn't bite her, but he is scared still then perhaps she threw the hay a little bit too close to him and he got scared. If he has trust issues then this could have been enough to set him off.
Just remember that sometime people like to make up stories to make themselves big, but when they find out that it's upset another it's a it hard to backpeddle and not be made out to be a fool.
Take a deep breath and chill, talk to her politely
and explain the situation, perhaps thank her for feeding your horse but can she please take more care around him.