If Frozen Fortress triggers, but no one is controlling it and the accountable player (the player who has the turn) doesn't acknowledge it, but the opponent does want to acknowledge it, does the trigger still happen?

No — the opponent pointing at it isn't enough; only the accountable player acknowledging it puts the trigger on the chain. Frozen Fortress is uncontrolled and its ability makes no player choose anything, so accountability falls to the turn player (CR 190.6.b, TR 506.1.a). If they don't acknowledge it before its point of observable impact, it's forgotten and never goes on the chain at all (TR 506.4).

The opponent isn't helpless, though — they're allowed to notify even though they aren't accountable (TR 506.2), and if the turn player then acknowledges, the ability goes on the chain normally and everyone gets a reaction window. If it truly gets missed, call a judge before the game moves on: the missed-trigger remedy gives the choice to the next opponent in turn order — here the player who wanted it — to resolve it immediately or miss it (TR 702.2.f.2), and deliberately passing on a mandatory trigger that advantages you upgrades to a Warning (TR 702.2.e).