Hey,

this is an understandable awkward issue. I don't think you are crossing any boundary with T at all, because you are not just befriending someone that you know is her friend for the sake of it and by facebooking her friend you aren't actually adding her- but at the same time It might feel too weird for her. If I was you I would talk to your T first and see what she says, tell her your concerns etc and maybe say to her if she feels more comfortable you won't add this mutual contact.

xxx