haha, trick is to get the bike before you get married, it smoothes things out.
My first 500r, told my parents that is was my friends. When my friend failed to pay me back the money I fronted him to buy it, I told them it was mine. I was forced to sell it or get kicked out of the house, but we had a good run.
My current 500r, my parents found out about when I got a parking ticket at school for parking in the dorm parking lot (I actually have a parking permit for that lot, on the bike) and the appeal information got sent to my mom's email address. A red 1994 "motorcycle" as they call it isn't exactly a blue Subaru, so oh well. They've given up on keeping me from having a bike though, so it worked out.
Your friend should clue in his parents ASAP, it will become a problem if he waits. It does depend on the family though.