Lots of NFJs in my life. My grandmother, who raised me; My girlfriend's mother, who I'm close with; a girl I adored when I was younger, but never got through to, who is now one of my best friends.
They all seem to have trouble with objectivity, not honesty. They say what they mean and they mean what they say, even if it's sometimes draped in metaphors and euphemisms.
However, they often aren't able to grasp that just because something feels right doesn't mean it is right. NFJs are the kind of parents who will say "Because I say so," and discourage skepticism in their young ones: The kind of parent-in-law who will be very skeptical and barbed about you at first, but then will accept you almost blindly from then-on: The kind of girl who knows you're not right for her despite the fact that you have similar interests, get along great, and don't find each other unattractive, yet won't be able to tell you why she knows such things.
NFJs are very passionate and protective about the things that orbit around them. Convincing them to be objective is hard, because if you do it on objective terms, they'll dismiss you. If you do it on subjective terms that they like, they won't truly understand objectivity. The best way for an NTP to handle them imo is to just be attentive, jokey, not too harsh on their lack of objectivity, and try to peer pressure them (yes, I mean it) into getting them to do what's right, if you think you can adequately conclude such a thing. They tend to be gullible. Peer pressure and subjectivity are their language when it comes to things they aren't super-experienced with or opinionated about due to experience. They are people people people.