This is terrible!!!
It is the fault of the sisters, who the girl trusted her child with
It is also the fault of the girl for not freaking out when the very same friends that she trusted her child with showed up at the party.
If it were me, I would have been causing a huge scene when they showed up and said "Oh, we put your kid to bed, but the door's locked"
Don't these girls realize that it's not always the things outside the door that can hurt the child, but the things inside too?