Who dont you believe? D14 or your STBX? It sounds like both are saying that D14's biggest issue is with friends which is typical for teenage girls.
Because of things D14 said to me, I don't believe STBX. I think STBX is blind to the real issues. Both D14 and D18 have told me during their 'venting' sessions with me that I act like a parent with them and STBX acts like a whiny sister.
So, all I _can_ do is stay the course and be the rock.