He walked away because he thought it was what she wanted. When FBI Agent Tom Drake arrives at the scene of a robbery, the last person he expects to see is his ex-wife. Seeing her again reminds him of everything he lost when they divorced, and the only way he’s going to survive being around her again is to keep reminding himself this is just another job.
She let him walk away because she thought it was what was best for him. Hannah Buffy is conflicted about seeing her ex-husband again, especially when he starts claiming he thinks the robbery at her store wasn’t random. Positive it is just another way for him to paint her as a victim, she ignores his warnings at her own peril.