They were told they would die if they ate it. Eve ate and didn't die. Thereby he had no reason to mistrust her when she offered it. Without knowledge of good and evil, he couldn't discern that God lied for his benefit or not, she might be lying, that they were fucked, anything.

The problem is, God considered Adam culpable (although less than Eve), meaning that he could in fact discern enough to preserve his power of choice.