Assume two Indians, A and B. There are four possibilities...

Now hold on a minute.

Did I miss something? I don't recall the question indicating that Indian A and Indian B knew each other, or more specifically which tribe the other was. (Maybe the tribes are very small, and everybody knows all the members of both tribes? But that isn't stipulated, either, or excluded. And if the tribes were visually identifiable, the questioner would be able to tell, too.) So neither one necessarily knows what the other will say, and the argument fails.

All of which is quibbling, of course, because we all know very well exactly what was meant, and all we're doing is pointing out the lack of perfection in phrasing the question initially, which [imperfection] is permitted. Or ought to be.