I would say you are free to comment if you would like to have a civil discussion about the issue, but you aren't "condoning" what she said by letting it go without comment.
I would say there are generally two types of political posters on facebook. The first are people like my friends and I who want to share what we feel are interesting perspectives on issues we care about, and who are interested in hearing what others think on the issue. Then there are people who are aggressively posting things and aren't interested in conversation at all. I had one of these types on my friends list, he would post things and when people would make perfectly reasonable comments in response, he would block or unfriend them. None of us understood it: why post something if you don't want to discuss it? I eventually unfriended him because of this behavior.
But I guess it all comes down to whether you believe civil conversations about politics, even with people who hold different politics than you, are possible and worth the trouble. I think they are, and enjoy them. I know others think it is never appropriate to discuss politics, and I don't start conversations with those people out of respect for their position.