If opponent has 1 unit on forbidding waste and i attack. On his focus he plays a unit into forbidding waste. Does the defend -2 still applied?

No — the -2 immediately stops applying. Forbidding Waste's "defending alone" condition is a static effect that re-evaluates the moment the game state changes, so as soon as your opponent's second unit is present at the battlefield, their original unit is no longer "alone" and loses the penalty (FAQ #10921; Rule 741.1).

It doesn't matter that the unit entered mid-showdown on their Focus — the penalty isn't locked in at the start of combat. Just make sure to use that unit's updated Might (base −2 removed) when assigning and dealing combat damage in the Combat Damage Step (Rule 460).