I love the deeper level of insight that comparing how terms or ideas are expressed in language, provides! Thanks - that's an interesting one. So going off of that, it just reminds me of how fluid it all is. In their opinion, then, maybe it isn't considered corruption. (That might not be the right example, but you can see what I mean about gray areas). For example, let's apply your friend's experience sort of in reverse. Someone could easily, when having first discovered a culture's habit of tipping tour guides or restaurants, called it a "bribe to receive decent service"--basically calling to our attention the fact that while we say 'tip' to make it sound normal, others might think it makes no sense--it's extra payment in order to "help guarantee" something that the business had already guaranteed based on the stated price or expectation.
In other words we might say the people in Nigeria used a word similar to Tip in order to describe something we would look down on by using 'bribe' (expressing our opinion of its corrupt nature), someone could also take some of our tipping situations and use the word Bribe to express their opinion on its corrupt nature.
It's one reason Spanish fascinates (and also frustrates) me. I get to see how they think of things differently! In many cases their wording puts the focus on another object rather than themselves. While both cultures might say, roughly equally, "That makes me mad".......Yet, if I arrive late (in English) I would usually say "I Am Late". But they might say "Se me hizo tarde" or "Es tarde", both of which make it seem more like something else 'got me late' basically. Funny stuff. There are much better examples but I can't seem to think of them now.