how does reinforce work with astral heron if its on a bf and is the first card you play

No — the unit you play off Reinforce doesn't get the Heron discount, and it doesn't consume it either.

Heron's trigger ("when you play your first card") only fires after that first card finishes resolving (Rule 419.4.a). Reinforce resolves by playing the banished unit as part of its own resolution, so that unit is already on the Chain and queued for finalization before Heron's trigger even goes up — cost determination for it already happened with only Reinforce's [5] reduction applied.

The discount applies to the next card played after Heron's ability resolves (FAQ #11762). So the unit from Reinforce plays at its reduced cost, and then your next card played later that turn still gets the full [2][A][A] off. Reinforce itself was your first card; the Heron discount is waiting for your second.