So I got into this discussion the last time my group met and I still don't have a solid answer. When a medium character wielding a lance charges on his mount he needs to stop and attack from 10 feet away to make his attack because he's using a reach weapon. This is where the argument begins for us, my friend uses the following passages from the Mounted Combat and Charge sections of the Combat chapter for support.
If your mount charges, you also take the AC penalty associated with a charge. (Link)
You must move to the closest space from which you can attack the opponent. (Link)
He points out that because your mount is the one charging and because it must move to the "closest space from which you can attack the opponent" which for a horse is adjacent, its rider can't make an attack with a lance or any reach weapon.
While I can't help but agree with him in his interpretation of the rules there are glaring examples (double damage with a lance, spirited charge, etc) throughout the core book that indicate a mounted character can indeed make a charge attack with a lance. Is he wrong?