For one thing, it violates the integrity of play; particularly from the scout's side of things when the scout's player would rather be left to figure this situation out on her own (and I've sat through the resulting at-table arguments enough times to know this is a thing). For another, it violates the integrity of playing one's own character from the rescuers' side of things, as you're basing your decisions on information the characters don't and couldn't have.
That's just it - I don't. Not if I'm DMing in any sort of good faith, at least.
The setting elements don't change just because more (or less) PCs show up than expected, or because they do things in a different or unpredicted way. If the scout's somehow run herself into something that was intended as a challenge for the whole party, so be it - she's on her own and probably up against it. She chose to go there and, either due to bad luck or bad management, ran aground; and now she has to find her own way out of the mess if she can.
If the trouble was already there to be run into, sure. But I'm not going to invent it on the spot just to deal with this; as if I do I'm no better than the metagaming players. I'd rather just cut the metagaming off at the pass and have done with it.