Durik's choice to move into an occupied square prevents him from using his heroic feat- them's the breaks.
Zaltyre - I'm not sure I understand your position here. Are you specifically talking about Durik moving from a space occupied by a monster into another space occupied by a monster and attempting to use his heroic feat on the first monster? That would ring true to me as I believe the errata says somewhere that when interrupting a move action, a hero/monster must be able to place his figure on the map and while in this instance there is no move action involved, I think it would be fair to assume this rule applies to all action interrupts.