Let's say Kruv has just failed his Hack'n'slash roll (6) against a goblin. Before the GM adds fictional consequences, Axandra try to do something to turn Kruv's failure in a (partial) success. She guesses that Kruv is confused, and the goblin is dodging to the the left. So she screams "to the left!". Is it possible? Is it mechanically correct to aid Kruv not before, but after his roll?
Strictly speaking, no, it is not mechanically correct. Hitting the roll +bond to Aid or Interfere only gives someone +1 to a roll they are shortly going to make, not a roll they've made already. The usage of "take +1" is even more immediate than "take +1 forward", and it's clearer in Apocalypse World where the help or interfere move adds the rider "with a character who is currently making a roll".
From an anti-exploitation point of view this makes sense. The roll carries some risk to the person making it, especially on a 6-. If you could just freely make the roll afterward, why ever bother to roll to help someone until they make their roll and you see if they needed it?
First, at table, the aid or interfere move can often be an interrupt to the normal course of play. As a result, timing on it is tricky, which is why the AW adjudication guidelines say:
It’s best if both the players — the acting one, the helping or interfering one — roll at the same time, but don’t be a nit about it.
But this is about timing in play, not waiting for results. Your aid action is taken to help the person as they're making the roll, not to mitigate any particular cause of failure.
Second, the rules are there for the GM to hack and otherwise make situational alterations to. It's a pretty easy GM shortcut, when you pull the spotlight off whoever just rolled a 6, to offer the next person a chance to roll +bond to help if it makes sense, often as a choice between that and addressing some other danger.
But in both cases, that's explicit GM permission at work, not player initiative in response to seeing a 6 hit the table. Remember, rolls only happen when the GM and the player both agree they should.
Extremely Online Gaming
If you're playing in an online environment where the results of a roll are instantly announced to everyone then there's no gap between someone rolling dice and everyone knowing the result. It's up to you how to deal with things in that case; when I was doing play-by-post I kind of split the difference. I asked people to roll when they acted and not wait for a whip-round of helpers, and if someone wanted to help on a 6 or 9 I had a standing policy of "sure, but you'll always expose yourself to risk on a hit" or "sure, but tell me what personally-important thing you're ignoring to help your buddy".
There's no restriction in Aid or Interfere about whether the aid has to be rolled first. It's more important, though, to begin and end with the fiction per the GM Principles. What makes more sense in the fiction?
Is it conceivable that Axandra's advice would help in this circumstance? Let them roll to Aid.
Say instead that Kruv is delicately disarming a poison needle trap. Here the outcome is fictionally immediate and unlikely to be helped by yelling. If Axandra wants to help, it's something they'd carefully want to describe in advance before making the roll to disarm. It would be a lot harder (but probably not impossible) to describe help after the roll.
Aid or Interfere
When you help or hinder someone, roll+bond with them. ✴On a 10+, they take +1 or -2 to their roll, your choice. ✴On a 7–9, they still get a modifier, but you also expose yourself to danger, retribution, or cost.