This kind of BS infuriates me. They should definitely pursue this with both US and by filing a formal DOT complaint when they return from their trip. I would specifically request of US: 1) Reimbursement for the flight they had to buy to PHL; and 2) Compensation for Involuntary Denied Boarding, as suggested above.

If you have the names of the agent(s) they dealt with, please name them very specifically in the complaints. Not that it will likely matter much, but they should at least end up with these complaints in their personnel records.

By the way, haggis, welcome to FlyerTalk. Sorry it wasn't a more pleasant situation that brought you here.