He should have done something. He shouldn't have stopped her, no, but he ought to have provided guidance. Formed her gift. Taught her boundaries, even as Buffy was teaching him to stretch beyond his own. Watching Willow grow, he only allowed himself to see the student. He flinched away from the witch.
In the end, he ran away, escaped the children who relied on him. Children they were still, despite their history, requiring direction and firmness to mold their strengths. Frightened children they were when he returned, flung open the door, and saw himself, Ripper, rage untamed in Willow's eyes.