It doesn't take much human intelligence to override the artificial intelligence that Intuit has built into the program.  The vouchers are a lighthouse guiding him to a safe harbor.  However, there is no indication that he is lost at sea.  If his income happens to increase substantially this year and his withholding doesn't keep up with it, then he can avoid the ES penalty by making quarterly payments.  Tell him if he sees that happening, he might consider making a payment.