I think you deserve to know what your best friend is not telling you. If you're thinking about entering into a serious relationship, you deserve to know ALL of the facts,...all of them. After you know the facts, then you can make your own educated decision instead of guessing along the way. If it's serious enough for your friend to cut off your friendship, then it's serious enough for you to know. This is your life, not a game, and your friend should tell you what the serious issue is that he knows about this person.