Friends employer refuses to pay overtime wages

Location: Pennsylvania

Friend of mine's employer does not pay any overtime wages. Any hours worked beyond 40 hours/week are paid at normal wages. For some employees, he has them "sign" away their right to overtime by making them "salary" employees, while still paying them in hourly wages not salary.

This has to be illegal right?

Anyone who does not sign a salary paper also does not get overtime, so not sure what the point is.