My preferred approach is that any roll that is "incorrect" in any way, is discounted entirely, regardless of situation or which faces are showing. This applies to rolling dice before required (as in the example, GFI can't exist unless out of movement). Rolling too many dice (rolling a 2db, which on examination is 1d). Rolling too few dice (rolling a 1db, which on examination is 2db). Many other examples exist.
The reason for this is that any other approach is subject to gaming.
For example, one suggestion above:
In our club we'd expect the player to take the 1 as their next roll
This is a poor suggestion, as it technically allows for gaming the odds. For example, if I was devious enough, I could "accidentally" roll GFIs regularly on my last square of movement prior to requiring an actual GFI. If I pass, then great, take the square, plus the GFI square. However, if I fail, I simply state that I never meant to move the player that far and (as per the above rules) know that I have to use the '1' for my next roll. I now know that my next roll is a fail, so I don't roll any dodges or ball handling rolls. I simply position my players and roll blocks, declining to use the '1' through the turn. If you absolutely force me to use it, I use it at the very end of my turn, on a pickup roll, or move my least useful player 1 GFI, as failing it is the least impact.
Similarly, rolling the wrong number of block dice can be used to game the odds if people don't disallow all dice. For example, I've seen some people 'accidentally' roll two dice for a one dice block. If the faces come up the same (say two Pows/DefStumbles) then they say "well it doesn't matter which dice, so I just knock you down", however when the odds are calculated mathematically, this actually improves their odds of a knockdown over the odds if they'd just rolled one dice. Further, there's no guarantee that they'd apply similar reasoning if two skulls were shown, unless agreed in advance. Using this method, anyone can enter a game, not agree this in advance, then use the "accidental" wrong dice roll to reroll all dice where disparate faces show until a double pow exists, then claim the hit. If a double skulls shows first, just say that it still needs to be rerolled.
The only safe way to correct errors in BB that doesn't allow for gaming of the system is to ensure that ALL dice that have any errors incorporated into the roll are discarded ENTIRELY. Anything else is subject to playing the system for advantage.
In summary, the roll wasn't for a GFI, so it wasn't for anything. Make him proceed with his turn.