That was a stupid thing of Schweitzer to say. Romney can't control where his father was born.
The Governor of Montana has gone down a point or two in my book.
In the context of the question Schweitzer was asked, what if Romney starts to pretend he is a Hispanic on the basis of his ancestors sojourn in Mexico, then pointing out why they were there would be fair game. Not that there is any evidence that the Romney camp will be so dense as to make such a foolish claim.
However, Schwitzer could and should have been more subtle. Here's a better way to answer that absurd hypothetical.
"Governor Romney isn't really a Hispanic. His great-grandfather had decided he didn't love the United States any more, so he left it for Mexico. His grandfather then brought his family back to the United States because he didn't love Mexico any more."