I like to think of myself as an emotionally intelligent person. I am always there to listen if my daughters have an issue. I'll generally respond in the way that they need at that time; so if they need a vent, I'll listen and not necessarily talk too much. I generally help them reach their own resolve, rather than wade in to solve their own problems!
My 17 year old has just come to ask my advice about something. This doesn't happen very often, so I was really happy to listen and give my opinion (which she asked for).
She ended up flouncing out, because I am supportive but won't just tell them what they want to hear. 
She is 17 1/2 and has been seeing her (first, steady) boyfriend for 7 or 8 months. Largely it's a loving, happy, supportive, healthy relationship. He's socially confident (until recently, boarded at private school) and she's quiet, anxious and a bit insecure. As far as I can make out, he tries his best to reassure. He seems trustworthy, loyal, respectful and honest.
Recently he has been communicating a lot with a fellow boarder from school, who now lives overseas. I think because of how my daughter can be, he downplayed the closeness and communication. My daughter has found out that they have been messaging each other a LOT, more than she feels comfortable with. She does trust him though.
I gently pointed out that if there's nothing untoward going on, perhaps he feels it necessary to cover things up (to protect her feelings). I said that the sneakiness is the bigger problem than his friendship with this girl, and that this is the crux of the issue. He's a non-toxic person who has girls as friends. This same thing could keep happening again, and it's important that my daughter works on her reaction to that.
She is adamant that she has no problem with him having girls as friends, but she sees him getting closer to this girl and there is nothing she can do about it.
I feel awful now, like I didn't really have her back. But I don't think a reactive response like 'oh yeah, he absolutely shouldn't be messaging her' is going to help anyone.
In my view, trust is important in a relationship until our partner gives us reason not to trust.
AIBU?