He's right, there were many non-Russians that fled to the metro. Some discriminate against them (Neo Nazis being the most obvious), others accept them as human beings and cooperate as such.

I'm wondering if America and Russia fought each other in the nuclear exchange or were at war. If they did, or if the countries were in tension, your character would probably have to hide the fact that he/she is actually American - but that shouldn't be a problem in a predominately Russian-speaking metro if they learned the language. Wouldn't have to keep it from all people of course - some Metro residents are all about peace. Others with narrower minds, not so much!

That's the only thing I could see wrong with the tourist, but if anything that might make things more interesting... ;D