If I ambush with Kha'Zix to defend against an attacking Rengar, then my opponent responds with Nidalee to ambush in alongside the Rengar, does the trigger still give +2 on resolution?

Yes, Kha'Zix, Mutating Horror will receive the +2 Might bonus and gain 2 XP.

Reasoning

According to multiple RiftJudge FAQs (e.g., #10198, #10735, #11155), the condition for Kha'Zix’s triggered ability ("if an enemy unit is alone here") is evaluated at the exact moment he enters the battlefield and gains the "Defender" combat designation.

Chain Sequence and Resolution

1. Ambush Timing: You play Kha'Zix as a [Reaction] to the combat using Ambush.
2. Trigger Condition Check: When Kha'Zix enters the battlefield and gains the "Defender" designation, the game checks the battlefield state: Is an enemy unit alone here?
   • Since Rengar was the only unit present at that moment, the condition is satisfied.
3. Trigger Placement: Kha'Zix’s triggered ability is placed on the chain.
4. Reaction: Your opponent plays Nidalee, Cat Form as a [Reaction] to the battlefield. This adds Nidalee to the chain above Kha'Zix's trigger.
5. Resolution (LIFO - Last-In, First-Out):
   • Nidalee enters the battlefield.
   • Kha'Zix's triggered ability resolves.

Because the condition was successfully met at the moment Kha'Zix gained the combat designation, the subsequent change to the battlefield state (Nidalee entering) does not retroactively negate the trigger. The game does not re-evaluate the "alone" condition upon resolution.