One of my degrees is a BA in Japanese; I studied it for 3 years in the states and was an exchange student in Japan for the 4th. Even at my best, I still wouldn't say I was ready to keep up with native students at the college level, although I was willing to try.
If you are talking about the special major, which I have too, yeah it isn't enough or even close. Your Japanese was always above mine though and probably still is. In many ways, I think it is entirely up to the individual how fluent they will become, rather than the degree or classes.
Honestly, anyone going to college in Japan who is from the United States in any degree that isn't directly related to Japan... I simply can't understand the reasons. US universities are simply going to be better in almost every case I've seen so far. Now, if your studies are directly related to material for which you need to be in Japan to study, then by all means, go for Japan. However, for computer science I really don't see the need.
For myself, my research is directly related to Japan and Asia; therefore, I aim for study in Japan. However, even during my undergrad study I completed my degree in the US and only used the study abroad programs to go to Japan (which I have found to be the best way to go about it).
"Study in Japan sounding neat so I might as well try to do a BS in Japan." Is not a sensible reason by any degree. That's like saying "Gee, I wonder how a nuclear bomb works... I think I'll learn Russian for 8 years, go to a Russian University build a bomb and set it off, instead of looking it up on Wikipedia or going to a museum."
Ok, maybe not the best example but it sounded better in my head. Actually, maybe that previous sentence best fits your friend's idea after all.