Return once more to the Light Side

In my campaign a level 13 paladin PC is about battle a blackguard. Eventually, the paladin will realize the blackguard is a former colleague who fell from grace. I’m sure the player will want to turn the blackguard back to the side of good again after the blackguard is defeated.

Can a blackguard regain his good alignment? If the blackguard does become good, what happens?