Bella eyes him a little, but the question, once she's untangled that sentence, is easy enough to answer.

"If someone is lying to you," she says, "you find out what and why. You're not obligated to trust someone who's not telling you the truth."

The part about depth of acquaintance reminds her, though - if she does decide to ask him, she'll have to do it soon, before they can form any kind of real friendship. If she waits too long, and they do become friends, revealing what she knows and that she went behind his back to find it out will very likely ruin things.