Can LeBlanc's Legend ability be used during opponent's turn? For example if they get charmed

Yes — Deceiver is a triggered ability ("When you conquer or hold"), so it isn't restricted to your turn; only activated abilities are (Rule 381). If the trigger condition happens during the opponent's turn, it fires and you may pay the cost then (discard 1 + exhaust her).

The most common way this happens is exactly your Charm example: the opponent charms your unit into a battlefield you don't control, you win that Showdown and establish Control, and you score a Conquer on their turn (FAQ #3426). At that moment Deceiver triggers, the trigger goes on the Chain, and you may choose to pay the cost.

Two caveats: "Hold" can only occur during your own Beginning Phase, so that half can never trigger on their turn; and you still need at least one card in your hand to pay the discard cost — with an empty hand the trigger can't be placed on the Chain at all (Rule 422.3).