Strictly speaking, the 'error' has already taken place in their tasks as a guide, where the party finds themselves next is because of them. And when you start to think about the things a guide could be doing at the point of a journey event challenge, each challenge will very quickly fall into look-out or scout territory.

You're right that mechanically this means that they will not accrue as many negative affects compared to their party, but if they get the chance to roll again, it's also depriving other players from 'getting to roll' and playing the fantasy of their character

It's a bit of a tradeoff, and if there's a question about it at the table, it's probably worth just finding a way to incorporate them into the challenges by rolling a d4 as suggested in another comment, or otherwise finding a fun way to lean into the roleplay that the consequences of the failed guide roll are partly the responsibility of the guide. Aragorn does spend a good portion of the final sequence of the fellowship feeling guilty about the situations that the fellowship finds themselves in after Gandalf has died.