Tyranid double-advance

I don't remember the name of the strategy, but the tyranids have a strategem that allows them to double their advance dice (when they are rolling 3 dice and picking the highest, it tends to be a big deal). Anyways. The wording on it says 'when you roll to advance, you may use this strategem' My opponent said that it doesnt specify that he has to use it before he advances, so he can choose to use it after he see's the results of the dice. Is that true?

Anyway you could find the full wording of the stratagem?
For example, for Craftworld and Haelequins, "Matchless agility" and "Warrior acrobats" tempers with advance but specify "instead of rolling for advance"
In the case of this Tyranid stratagem, it says "when" not "after". I'd say that once the player rolled the dice and have seen the result, the roll is past.
In the same way, if a stratagem give +1 to hit or -1, you would not roll the dice then apply the stratagem modifier. You trigger the stratagem then roll

found this wording online, so dunno if its completely accurate, but...

Opportunistic Advance (1 cp): Use this in the movement phase. CHoose a KRAKEN unit that does not have the Fly keyword. When Advancing, you can double the number you roll when determining how much to add to the units movement characteristic.

That is their Chapter Tactic... Roll 3d6 and pick the highest.

Then there is their specific stratagem to allow them to double their advance.

As to the stratagem, I can't seem to figure where I put my codex... will check another friend's today.

That's pretty close. The actual wording is:

Use this Stratagem in your Movement phase when you
roll the dice for an Advancing KRAKEN unit (other than
a unit that can FLY). You can double the number you roll
and add that total to their Move characteristic for that
Movement phase, rather than following the normal rules
for Advancing.

I think I could argue either way on this one.

Unfortunately you will be looking at a semantics/grammatical debate for either case, which I never enjoy.
If pushed, I would argue that you have to declare at the same time (or before) you roll the dice, due to the wording of 'when you roll the dice'.

I would probably just let the dice gods decide if this came up during a game.

I would argue honestly that if you opponent roll first, it becomes a reroll. The Chapter trait and stratagem only say roll.

This is odd, i play nids and mostly only Kraken, but i never have more than 1 unit i need to use it on so i never really notice the wording.

Well its clearing NOT before, its not when you choose a unit to advance, it is when you roll, so for me i would say "B.c its not when you choose to advance, for before you roll, that you can choose to use this stratagem after you roll"

And i am not saying this b.c i play nids, it will never come up for me, literally never will,s o i dont care either way, but this is just how i see the rules and grammar.

PS: Is there any other rules that says "when you roll"?

Well since GW is never ambiguous, the wording clearly indicates that it's not before the roll OR after the roll, then obviously the Nids player is expected to use the strategem during the roll, when the dice are in mid-air!

Makes 100% sense!