Don't even bring up the fact that you know that he's been having behind the scene conversations with her that are clearly crossing relationship boundaries.
Simply google "Emotional Affair" find a good read on it, print it out and then bring it to him and tell him that you don't want him getting emotionally involved any further with his female friends and would he kindly stop his online communication that crosses said RB's.
Don't let him turn it around on you and changing the whole tone of the discussion by making this about snooping. Tell him your fears that the more he becomes dependent on her and their convos, the bigger the emotional gap that you and he shares will become and more bonded he will become to her. Your printed out article will back up your conclusion.
Be confident and straight up about your anxiety to him and keep the conversation focused on his attachment to her and off of you "snooping."
He WILL defend his actions and he will, more then likely accuse you of being overly jealous and or down play his "friendship" with her. Have your facts ready about what can happen when he, himself crosses relationship boundaries and doesn't distance himself from the slippery slope he's started down.
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I'll add that you could have sex three times a day everyday and the "spark" be sparking till the cows come home. If someone is crossing relationship boundaries and not backing away from people that are crossing said boundaries then what is happening would still happen. As long as he and she are still acting inappropriately and with disrespect to you and your relationship then trying to do anything with him to get back a spark that MAY be missing will fail.
BTW: Just because you are in a relationship it does not give you the right to indiscriminately look through your partners things. Snooping by the mere definition of the word means you have violated someone else's privacy. We do not give up our individuality just because we are in a relationship. Snooping when there has been shady and suspect behaviour going on is one thing. Thinking you arbitrarily have a right to invade another's privacy just because you are romantically attached to them is quite another.
Last edited by Wakeup; 10-04-15 at 08:21 AM.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion