Ask him more specific. If your friend can't give a more specific answer then .. well.. his knowledge seems to be somewhat limited. Read the reviews, you can start here: http://www.photozone.de/canon-eos
In perspective, the multiplied focal length gives you a roughly the same focal length. But the f stop is wider, but i think the AF and sharpness of the 15-85 and 24-105 really very similar. Even though one not L lens, but it's sharpness is nearly L lens quality.
Ducky, a lens/camera is a tool. It's how you use the tool. You need to know what you're shooting for the lens to be good. Your friend needs to learn up a bit more before shooting his mouth off. FOw example, I can give you a 16-35L to pair with the 550D and i will say it's a bad lens for me since I need something in the 70mm range rather than wide for my normal street shooting.
IMHO I think both no good, because I like ultra wide and i also like to zoom ..... and i also like macro, so my lens will be macro ,..... so it depends on yourself dear TS. This is like asking people what they like to eat ..... it have no bearing on what you like to eat...
If can only affort 1 lens and have to choose between them, my perference will be 15-85mm.
Without wide enough angle will have problem when in tight spot. Without the extra reach can still capture distance object and crop it. Of course quality will be sacrificed and perspective will be slightly different.
Well, there's no such thing as good or no good. It depends on what you shot. Your friend is somewhat right. I suppose you are a beginner, 24mm on a crop-sensor camera would be real tight (around 38.4mm). 38.4mm is really tight for landscape work and acceptable for perhaps walkabout or product shots. The 24-105 is definitely a better lens, in terms of its built and image quality.
Since you are still a beginner and has no idea what to shot (my assumption), I think you should not restrict yourself first.