This is a great question and could conceivably be put in the Baseball Leftovers for more discussion. Mods, think about it.
You would need to try and determine your friend's dedication and love of baseball. I find that if I only had a passing interest in baseball, or fantasy baseball for that matter, a keeper league wouldn't work out. You have to be willing to know information about most of the players on every team as well as their minor league system. You also have to be prepared to know the difference between parks, divisions, and manager tendencies. If this friend is merely a draft-and-watch kind of guy with minimal interest in trading, then he might be a tough addition.
As for advice to join a keeper league, tell the friend to study depth charts. He should also consider picking up a few issues of Baseball America to get the idea behind prospect watching. Finally, a few tips should be related to the scoring of your league. Are pitchers or batters more important? Are there too many categories to worry about just saves? If it's an auction league, is it better to spend money or go cheap? You can reveal as much or as little (competitively) as you wish, but through these explanations the friend will know what he's getting into. Even though it sounds like he can be scared away, I bet it is equally if not more likely he'll be more enthusiastic to join.
You may also want to remind him that in a keeper league having a player on the roster carries more weight than a redraft, so consider options more carefully regarding trades and drafting.
I hope this helps.... er, got a moment?http://www.fantasybaseballcafe.com/forums/viewtopic.php?t=420326