WRT the trip, I am just going to approach her and say "Look, W, I really, really want to do something with you on your birthday, and I had wanted to surprise you, but bff's overtures are making that very hard... I really wish she had contacted me first. I'd like you to keep that weekend open".
Please do not do this. It will not be a nice gift or something she looks forward to if you make mention of her BFF. Instead it makes it something stressful that makes it feel like she is being put in between. I notice a lot of you what you say to your W includes a "wish you would have done this instead of that" sentiment. Leaving everything on her shoulders and feeling like she has done wrong which equals pressure, which takes the enjoyment out of things.
I love C-nut's expression. Here is a way to be confident of your decision. Don't put the onus on anyone else. Make the plans!
