I have a Fiora, Peerless sitting at a battlefield with Master Yi Wuju Bladesman as my legend. Oppononent moves in with their unit initiating a combat. Fiora triggers her ability and doubles her might to 10. If I ambush another unit in, will fiora lose the plus 2 passive from yi will she be at 8 might after?

Yes — Fiora ends at 8 Might.

The doubling already resolved, so she gained +5 (her 5 → 10) and that bonus is locked in for the combat. But Wuju Bladesman's +2 is a conditional passive ("while a friendly unit defends alone") — the moment your ambush unit lands, Fiora no longer defends alone and that +2 falls off. 3 base + 5 locked double bonus = 8.

This mirrors the FAQ ruling where Fiora at 7 was doubled to 14, then dropped to 12 after an ambush removed Yi's +2 (FAQ #11172). For contrast: if you'd ambushed as a Reaction before her trigger resolved, the +2 would have fallen off first and she'd only have doubled to 6.