Apologies for the potentially long post, but background needed for context - I'll try to keep it brief and to the point!
Our 12.5yr old DD1 has ADHD and poss undiagnosed ASD. She is incredibly bright and does very well at school, but at home is prone to violent mood swings, very heightened emotional responses, obsessive behaviours, temper tantrums and argues with us a lot. She also hates discussing feelings and consequently bottles a lot of things up.
We recently checked her phone (this was a condition of her having a phone, not me being nosey but just checking to make sure everything is ok) and there were messages with a girl who used to go to to her school, they were friendly but lost touch when she went to another local school. To cut a long story short, my DD has told this girl that she is bisexual, and the other girl has asked her out, my DD has said yes and has now come out as bisexual to her close friends via WhatsApp.
I don't have a problem with any of this at all, apart from the fact that she hasn't told us. We know she wants to meet up with this girl in town (who we've never met, but the messages say she's recently been excluded from school which makes me nervous) but we've told her that we'd like to meet her first. This hasn't happened yet due to the girls phone being confiscated this week because of her exclusion from school.
So my dilemma is this: do we just pretend we don't know about her sexuality, she seems to have forgotten that we occasionally check her phone! I understand that she would be nervous about telling us, as she can be very secretive about feelings she finds hard to deal with, but as her mum it hurts that she feels she can't tell us.
And do we let her meet this girl? I have real concerns about her suitability as a friend/girlfriend as her Whatsapps are very sweary and one photo looked like she had self harmed 
If it was my neurotypical DD2 I would feel confident that we'd be able to talk to her about all of this in a rational way, but DD1 is so emotional and angry a lot of the time I'd be really worried that it would end badly, and she would resort to lying to us.