The issue of atheist / agnostic is one thing. How to approach someone who made the statement you reported to me is quite another ... and perhaps not one best discussed in a work environment.
At some point or other, you might just say, "That issue you brought up to me the other day - did you want to talk about that sometime?" Keep it simple and non-threatening and open, and when and if she is comfortable talking about it in further depth, she may let you know.
As it comes to Eric Whitacre's music, I still think one of the best performances is the first, by the BYU Singers under Ronald Straheli. The CD that really got Eric big attention was the Hyperion release with Polyphony, "Cloudburst." Eric himself has conducted two CDs, "Light and Gold" and "Water Night," that second released yesterday in coordination with Virtual Choir 3.0. "Water Night is notable for its inclusion of a number of orchestral works, including settings of Eric's choral pieces for orchestra.
I haven't even mentioned Eric's work for wind ensemble, which would include his Ghost Train Triptych and his tongue-in-cheek Godzilla Eats Las Vegas, but we can deal with that another day!