I admire this young co-worker for her concern. She shows more maturity than her more seasoned co-workers. I think she should continue to be pleasant to the courier, not raise her level of friendliness, because it definitely could arouse the courier - but on the other hand - it may lift his spirits; something that probably doesn't happen to often for him. I'm not a guy, so I don't know. So keep it safe, and just be pleasant.
As far as dealing with her co-workers, I like the response "I never noticed that..." Referring to the couriers stares, or smells. She would be being truthful, because she hadn't noticed any of the older co-workers complaints about him. At the same time, if the co-workers are smart - they'll be enlightened by her maturity in engaging in gossip and being so petty. Maybe one or two will choose to mimick her behavior. But beware, there's always a "Negative Nancy" in a group, who will dismiss this woman's maturity, because seeing oneself in a bad light has a tendency to bring out the worst and she may instead use comments like "Oh, you're too young...you haven't been around long enough...." not grasping at the opportunity to just become a better person in general.