Can restaurant keep tips?

My friend works as a waiter for a national restaurant chain. When a customer puts their tip for her on their credit card, the company gives her the tip but reduces her paycheck by that same amount. Cash tips she can keep and her paycheck doesn't get dinged but the tips that are charged the company uses to offset what they would pay her. So, the more people tip her through credit cards, the smaller her paycheck becomes until it hits zero. She gets the credit card "tip" money but the company uses it as payment towards her base "salary". Is that legal? We are in Northern California.