I think it really depends on what her intentions were when she was hanging out with this guy. Was she doing it thinking "I'm getting away with this" or was she just looking for some conversation and maybe a little ego boost? Most of the women I know need attention in some way---if they aren't getting it from their boyfriends, most will seek it from other avenues...flirting with the cashier at the supermarket just to see if he'll flirt back, etc. If she felt insecure because you weren't giving her enough attention, and she went out with this guy never planning to be dishonest, and also never planning on doing anything with him, maybe she just needed to feel like someone wanted her, appreciated her etc.
I don't think she really "cheated" on you unless she did something physically with this guy, or told him she also had feelings for him. I say you talk it out, but let her know this is a one time chance, and next time if she's feeling like she needs attention, she needs to come to you for it.