Muslims love to quote Bart Ehrman, because Ehrman criticizes the New Testament. Of course, if Muslims would pay more careful attention to what Ehrman says about the New Testament, they'd realize that his claims refute Islam in two important ways.
First, Ehrman believes that the New Testament is extremely reliable. While he holds that it hasn't been perfectly preserved, that it contains contradictions, and that it isn't the inspired Word of God, Ehrman maintains that we can gather a ton of information about Jesus and early Christianity from the New Testament. Significantly for Muslims, Ehrman says that Jesus' death by crucifixion is indisputable, which thoroughly undermines Muhammad's position.
Second, if Muslims were to apply Ehrman's standards to the Qur'an, they would realize that the Qur'an fails miserably. The Qur'an hasn't been perfectly preserved, it contains numerous contradictions (by Ehrman's standards), and it is far less valuable as a historical source than is the New Testament.
Why, then, doesn't Ehrman criticize the Qur'an and silence the Muslims who appeal to his works? As he admits in the following clip, he doesn't criticize the Qur'an because he's worried that Muslims will kill him if he does. In other words, just as Trey Parker and Matt Stone will gladly mock Christianity on South Park, but won't dare mock Muhammad, so also Bart Ehrman will happily criticize the Bible, while remaining silent on the Qur'an. Why? Because Muslims slaughter people over the Qur'an, cartoons, etc., while Christians don't. Having the most violent religion in history has its privileges.