Who's allowed at supervised visits?

The answer to your question depends on the orders of the court. If the court ordered supervised placement and  in addition ordered that no other persons be present besides the dad, the supervisor and the child, the girlfriend has no business being there. If on the other hand, the order is silent on the subject, while you may disapprove it from an emotional standpoint, the father is not necessarily in violation of the court orders by bringing his girlfriend with him. If you believe the girlfriend poses a physical or emotional threat to your child, file a motion with the court to ask for an order that prohibits the girlfriend from being present during dad's periods of placement.