Does Fiora Peerless ability trigger if your opponent attacks alone first then plays Rengar Pouncing?

Yes, Fiora, Peerless’s ability will trigger in this scenario.

According to RiftJudge FAQ #1102 and #1231, Fiora, Peerless's "When I attack or defend" ability checks the "one-on-one" condition only at the exact moment she is designated as the attacker or defender.

The Sequence

1. Designation: Your opponent designates Fiora, Peerless as the attacker. Since she is attacking alone, the battlefield is one-on-one at that exact moment.
2. Trigger: Because the one-on-one condition is met at the moment of designation, her triggered ability is successfully placed on the Initial Chain.
3. Reaction: The game state is now Closed. Your opponent then plays Rengar, Pouncing as a [Reaction]. Because Rengar has the [Reaction] keyword, he can be played while the chain is already closed.
4. Resolution: Rengar, Pouncing enters the battlefield. Then, the Initial Chain resolves (LIFO - Last In, First Out). Because Fiora's ability uses "when" (a triggered ability) rather than "while" (a passive ability), it does not retroactively check the condition upon resolution. It will resolve and double Fiora's Might, regardless of the fact that Rengar is now also on that battlefield.

In summary, because the trigger condition was satisfied at the moment of designation, the bonus is guaranteed once the trigger is on the chain. Adding Rengar to the battlefield afterwards does not negate the trigger.