I the episode cat and mouse Ian Harman was a fushsbau and a leader in the resistance. So when Renard met with the resistance and said he understands wesen better then anyone there error Ian (if he is alive) would have also.
Renard says he has a Grimm I think the Rosalee could still find Ian and Nick would (likely) trust her more than Renard so she could ask Nick to help Ian or the other way around.
Looking at this could Nick connect with the resistance (one he trusts) be the group with Ian and Ian also has a reason to trust Nick? Could this link be better for Nick than Renard's?
Renard says he has a Grimm I think the Rosalee could still find Ian and Nick would (likely) trust her more than Renard so she could ask Nick to help Ian or the other way around.
Looking at this could Nick connect with the resistance (one he trusts) be the group with Ian and Ian also has a reason to trust Nick? Could this link be better for Nick than Renard's?