I (20F) have been best friends with N (20F) since elementary school. We went to the same school since elementary all through high school. Right now we go to different universities but it's in the same city, so we would still hang out all the time. That is until a couple of months ago.
I've been dating my bf (22M) for a little over a year and things are good for us. He's a very outgoing person and he has a few close female friends. My bf is very open with all of his friendships with me, male or female, and he always prioritize me first over his friends. I also think I'm quite secure with myself because I've never felt jealous or uncomfortable with him being friends with girls.
My bf first started being friends with N when he asked for N's instagram because he wanted to ask her about some food & drinks that I like so that he can surprise me with food deliveries every now and then. N has been dating her bf for almost a year and he works as a line cook in a cruise ship that's been sailing for 6-ish months. N and her bf have been having trouble communicating due to time difference, his long work hours, and limited internet connection on the ship. I found out a couple of months ago that she now has been talking to my bf quite often. She'd tell him about her day, stuff that she's going through, complaining about her bf, etc. My bf did tell me that N has been texting him but I didn't know it'd be this often. My bf has showed me the messages and his responses have been straight-forward and he'd rarely give replies that would make the conversation continue longer. N is the one would reach out to him first. I think the main reason why she's doing this is because she can't talk to her bf about it and she wants to have a person talk to about those stuff immediately. Any updates that I have on her life now comes from my bf first because she'd tell him first than telling me.
I felt upset when I first found out she's been confiding to my bf instead of me, her best friend since elementary school. I've told her that if she ever needs someone to talk to she can talk to me (even though I don't think that's even necessary because we've known each other for forever and I think it's obvious enough that she can always talk to me, like we've always been doing). I haven't talked to her about this because I'm scared this will ruin our friendship. Instead I've been keeping some distance from her and have been using schoolwork as an excuse to not hang out as often as we used to. What do I do?
TL;DR: My best friend has been talking to my bf about updates on her life instead of me. Her own bf works on a cruise ship so it's difficult for her to talk to him. I feel uncomfortable and upset, but I'm scared our friendship might be ruined if I call her out on it.