Factual error: Ruthie gets impaled on a jagged shard of glass from a modern shower door, but shower doors are made of safety glass (since 1977) - when they break they don't leave sharp knife-like shards, only tiny pieces.
Factual error: When Lily is being shot by the arrows, the first arrow just lands nicely in her chest, but when she falls out to the balcony after the second arrow is shot, they have gone all the way through her body.
Answer: If you remember, Dorothy kind of stumbled into her friend from around a corner before they both fell down the stairs. Adam could have quite easily forced her into the costume and then pushed her, making her seem like she was trying to attack her friend.