The group.
If the thief's player knew that keeping the passage secret would mean the paladin's player would be excluded from the prepared adventure, no amount of pleading "it's what my character would do" will change the fact that he's being a jerk, and is at fault.
If the DM, seeing how things went down, did nothing to help get the paladin involved or give him something to do FOR THREE HOURS, he's at fault.
The paladin's player should probably have communicated that he wanted to be involved instead of just sitting by grumpily reading books FOR THREE HOURS. Of course, I can only blame him so much given that D&D places most of the power to effect his involvement in the hands of the DM.
The wizard's player could have tried to help find a solution to get the paladin involved, instead of ignoring that player FOR THREE HOURS while he enjoyed his escapade with the thief.
That the party consists of PCs with such incompatible goals is an overall group failure, and should never have been allowed to happen. Granted, in D&D, it's primarily the DM that should have taken some steps to fix this initially, so he's more culpable than the others.
So, IMO, everyone at the table screwed up.
EDIT: Yeah, what Gold Roger said.