In light of YDK's answer here, Halacha permits one to greet and sometimes return greetings to other people at different times during prayer when other interruptions would otherwise be prohibited. (See ...
If you wash netilat yadayim and then a non-Jew says "hello" to you, should you: say "hello" back and wash again? try to acknowledge him without actually talking? or ignore him (doesn't seem right)? ...