Who must be the target of a golden opportunity?

So, a player is Aiding someone else and he gets 6-. Can the GM strike with a hard move the one who was receiving the aid?

That is: must the target of a golden opportunity be the one who rolled it?

When I take a golden opportunity, its to make a GM move, as hard as I like, against who I like, or against no-one in particular. There is no 'rule' on this. Your moves are your tools to stick to the agenda and instigate your principles.

So yeah, if you feel that the best move is to separate them, or take away the resources, or deal damage, or put them in a spot, or show a downside to the class/race/gear of the aided party, then by all means do so. Then turn to the other player and ask 'So what do you do?'

well, thank you :) for some reason, I was not sure about this thing, even if it does make sense.

This was something I didn't "get" at first either. It's not a tit-for-tat... you don't have something happen to YOU as a result of YOUR miss. It's just a turn in the fiction. As long as it follows, whoever is on the receiving end of that misfortune is up to the vagaries of context and The Cool. It might be you; it might be your buddy. Or someone/something else entirely.