What happens if star crossed the nonambushed unit in reaction to ambush

Yes — you can play Star-Crossed there, and it bounces your chosen friendly unit plus the pre-Ambush enemy unit; the Ambush unit stays and your opponent keeps the battlefield.

You can't react to the playing of the Ambush unit itself (it finalizes and enters immediately), but once it's in play you can respond in the resulting Closed State. Star-Crossed just needs a friendly unit and an enemy unit as targets — the pre-Ambush unit qualifies. On resolution, both targeted units return to their owners' hands. The Ambush unit was never chosen, so it remains on the battlefield, and since your opponent still has a unit there they keep control and keep their Defender designation (Rule 187.4.c). Control isn't lost just because the bounced unit left — the Ambush unit holds the location for them.

This matches RiftJudge FAQ #10057 exactly.