It's true that because German doesn't saturate our popular media the way English does yours, more of us English speakers who do try to learn German may be 'language people' -- people who are, if not professional translators or teachers, at least maybe more interested in, and attentive to, language for its own sake than the average person on the street. And I think the flip side to that is that it makes us more cautious, which isn't necessarily a good thing when learning languages. I have the feeling that we (well, at least I) probably sound too formal or too bland if we don't dare use colloquial expressions at all unless we're 100% sure they fit.
For example, when in English I would probably say something like 'Good grief!' or 'Geez, give me a break!' or 'Oh, come on, that's really stretching it!' or 'Wow, that's quite an accomplishment!', in German I often end up saying something duller, more generic, less nuanced, or more tentative. Like, say, 'Meine Güte,' or 'Ähm, bist du dir sicher?', or 'Gut gemacht.' It's frustrating. Yes, it's better to err on the side of caution than to make a fool of yourself, but on the other hand, playing it safe can sort of strip us of personality.
But to understand which colloquial expressions sound okay and which ones don't, I'm not sure geographic dialects are the best comparison. It seems to me that what some German speakers may not be aware of is that the 'Wassup!' kind of speech does indeed come from black American culture, which is why it can sound distinctly out of place in the mouths of whites -- at least, white non-rappers.
I wonder if it would help at all to compare it to something like Kanacksprache (sp?) that's also (originally) associated with an ethnic minority or with lower socioeconomic status? In both cases, if I'm not mistaken, middle-class teenagers may pick up some aspects of it to try to sound cool, but it would still sound much stranger coming from a grownup. Maybe that would help eric's friend understand a little better how it comes across.