A couple of other things.
1.
Japanese comics portray women in a bad light often. Foreign women are especially depicted as having huge bustlines, blond hair, and blue eyes, all of these being things that are completely different from what you would typically see in a Japanese. The result is the false allure of the foreign female. Even if you don't think your own daughter fits these physical characteristics, Tetsu may think otherwise subconsciously.
Also, J comics do not have a good image of women in general. Lots of sexual violence. Heads up. Who's to say how much of that, plus the typical J lifestyle leads to pent up feelings (not violence necessarily, but at least lust without emotion).
2.
Tetsu is likely trying hard to cope with the language barrier in school. Of course, people need social lives, but living in the U.S. is probably not the same as he did in Japan. So, he is being torn in all directions, academically and socially (and now emotionally, too). My point is that his parents are probably very desperate for him to succeed in school, and it is probably very hard for him because of the language barrier. Make sure your daughter understands that he needs time for that. In Japan, one's success in the business world depends on getting into a good college and graduating. Only in the last year or so has GPA been introduced here, so Tetsu will find it harder than usual when he returns, because students (and staff) are not used to GPA. The job market here is also dismal for J grads (30-40% couldn't find jobs in 2010 or 2011). This is enormous pressure on Tetsu (or will be). If he gets married early... see below.
3.
I don't think I have to write this, but I will. Getting married as a teen is an enormous life risk. In Japan, the marrying age has increased for males and females (over 25 now). People are refusing to tie themselves down, and instead they just want to enjoy life. Getting married is tough, and being married to a foreigner is tougher in some cases (personal experience here). Your daughter needs to experience what Japan is like before anything else. This infatuation will not be easy on her logical processes, so go with the flow. I've seen too many people who want to come here and "live all their lives" even if they've never set one toe in Japan before. Pretty stupid and dangerous. Try doing that with a foreign spouse in tow.