Is saying "these people are from Hell" considered profanity at the workplace?

My friend was at work,(he works at a call center) and after many unsucessful sales calls he said to himself “these people are from Hell”. Later he was told that he used profanity on the sales floor and could be terminated. His statement was heard by a manager that was monitoring his calls, but not by a client. It was said while the next call was being dialed.