To me the idea sounds great, but as a DM I think I would not be able to handle it properly. I would be stuck at the first problem: how much to let the main player know?
On one side, I always wanted my players to play the character they want. Just as I never forced them to play a Sorcerer or a Halfling (just to mention a few things that a PC is just born with, but we let the player choose), I would not want to force a player to have his PC turn evil, and not just evil but rather the BBEG! For this reason, I would choose carefully who of my players might find the idea interesting. IMHO it is necessary or the player may be very frustrated if the DM chooses for him.
Then, if he chooses to accept your opportunity, you have to make it clear that it almost certainly means his PC is going to die, killed by his ex-friends of course
. Even if he's not killed, he is removed from play in one way or the other (unless you want to split the players' group in half from now on).
Perhaps you have a player that may like that, maybe because he is willing to make a new PC after this events. The only risk is that you can only ask ONE of your players, otherwise if the first refuses he already knows what you are up to...
