It's not mandatory that a PC must attack every single round of combat. Under normal conditions, without penalties at all, a PC can opt to go full parry and simply not attack that round.
If a person is dazed, stunned, fumbling, recovering from a slip-up, whatever that case may be, he may want to attack that round, but just misses an opportunity to do so.
As for the text as written in the book, I think maybe it's case of "not the best word" used in the description. I would argue that text should have read "... can only parry a foe he is attacking..." or "... can only parry a foe he is currently interacting with or focused on..."
The PC is otherwise engaged in combat with the NPC and therefore able to parry attacks from that NPC. The PC is not trying to parry an attack from an NPC who is fighting a teammate.