Z400 Central is down right now. Where is your friend located? What's the elevation? There are some other guys on here who can probably help better than me...but I can give you a basic starting point if you provide the info. He may not be able to go up too much w/stock filter and the airbox lid on though. Why didn't he get an aftermarket filter....that's almost always done w/a full pipe.
Well, with an aftermarket filter, I would say a 155 main. Being that he has the stock filter, he may want to start with a 150 or so. Give Z400central a few minutes and post over there. There's a few people from KY that should be able to help a little better.