How do I confront my friend about talking to a girl I'm talking to?

I started talking to this girl a week or so ago and things are going well. Then out of the blue she stops talking to me for a few days. I share a computer with my friend and he left facebook logged in and I saw where he was talking to her. Nothing flirty, but why would he go out of his way to message her? Also I asked him about it and he made a point of saying she messaged first (She didn’t) and that it was innocent. How do I confront him and tell him to knock it off? or should I just accept that my best friend is snooping behind my back?