A third vote for the UUs.
Especially for an ardent and vocal atheist (I am an atheist and feel very at home in my UU church). Our church spends a lot of time considering questions of morality and philosophy. Sometimes using religious texts as one point of reference in the discussion, but looking at the subjects from many perspectives and not through a theistic lens. Different congregations do have different flavors on this, though. My oldest D belongs to a large UU congregation in Washington DC with several pastors, and when I went to church with her I noticed that some of the pastors were more "theist" than others. So if she wants to try it you might plan to go to a couple different ones if there are choices in your area.
I also attended my first Mensa meeting recently and there were a few teenagers there. I chatted with them, and they seemed like the types who would enjoy a good philosophy discussion!
