If I attack into Reavers Row and my opponent flees his 1 unit there, does he still get an action window?

Yes, the defending player still gets an action window even if they retreat their only unit from Reaver's Row.

According to FAQ #5125: "The Showdown continues through all its normal steps even if one player no longer has units present. The defending player can still play Actions/Reactions as if they had units there."

Sequence of Events:

1. Combat Initiation: The attacker moves a unit to Reaver's Row, triggering a Combat Showdown.
2. Initial Chain: "When I Attack" and "When I Defend" (including Reaver's Row) triggers are placed on the chain.
3. Resolution: The chain resolves in reverse order (LIFO). If the defender chooses to retreat their unit when the Reaver's Row trigger resolves, the unit moves to their base.
4. Action Windows: After the initial chain fully resolves, the state becomes Open. The attacker gains focus first, and players alternate playing actions and reactions until both players pass consecutively. This process continues through all normal steps of the showdown, regardless of whether units are present on the battlefield.

Key Nuance:

• A showdown does not end simply because a unit leaves the battlefield. It proceeds through all its normal phases, and both players retain the ability to play actions and reactions as long as they have focus and priority.