If my opponent play a hidden card and i defy it. does heron effect trigger and my opponent get to play 2nd card for 2energy less

It depends on what the hidden card is — if it's a spell, no; the countered card never counts as "played" for Heron.

Defy only counters spells, so if the facedown card is a unit or gear, Defy has no legal target at all: the permanent leaves the chain and becomes a game object (Rule 359.2), which isn't a spell. In that case it resolves as their first card, Heron triggers, and their next card does get the [2][A][A] discount.

If it is a hidden spell, Defy can legally counter it (Defy checks printed cost — 4 or less energy, no more than 1 power — not what they paid for it from facedown), and then Heron never triggers: a countered card is not considered played for abilities that trigger on cards being played (Rule 426.1.b), and Heron only fires once its card completes resolution. Their discount never exists, so a second card that turn is played at full price — and if that second card is the first one of theirs to actually resolve, Heron triggers at that point and the discount lands on the card after it.