this is a danger to your relationship. look up emotional affairs, find as much info as you can, learn about them and what they are and then approach her when you know what you are talking about. it may be helpful also to print some stuff off and ask her to read it.
approach the situation calmly at first, sit her down, talk to her, tell her that you feel this is crossing a boundary in your relationship and it is one that could cause you to break up if you cant sort it out now and figure out why she feels the need for so much attention from this guy? tell her your not sure if its a good idea to get married considering there is a third party in your relationship that she spends more time with online and ofline than with you. ask her is there anything missing from your relationship that has caused her to seek attention elsewhere. tell her you see this as a huge red flag and a threat to your relationship and you would like relationship counselling before you marry her as this is creating many doubts, insecurities and trust issues for you. tell her you love her and your willing to do what it takes to make your marriage work but you need her to put you first and stop this affair now if you are gonna have a future together
if that doesnt work-if she gets angry or defensive or trys to say your paranoid etc-then you need to tell her the wedding is off and your moving out. if the thought of losing you is not enough to make her stop this-then your relationship is already over
best of luck. keep us updated
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