I don't think notifying her firm is good enough. She should have to be notified individually, directly, and provably for them to get out of refunding the ticket.

It wasn't entry to MSG, but entry to radio city music hall which is owned by MSG. It's not clear how obvious the warning was.

If they have the tech to identify everyone individually they don't want going to their venue, then it's on them to notify everyone individually ahead of time. Otherwise, refund + expenses.