Most Helpful Girl
Not to be pessimist or anything, but it doesn't sound good. It doesn't sound good the moment she's telling you one thing and she's telling the other girl another thing. It's compeltely uncool to plan a beach getaway with your ex when the partner is gone. It spells CHEATING all over my board. It's bullsh*t they're just friends. If that other girl keeps on feeding your girlfriend ideas about the relationship being "stupid" and of you being a "downgrade", it'll get to a no return point.
I think your grilfriend needs to be clear about this. First, she doesn't complain about your relationship to others unless she first complains to you. Anything the relationship might need is only between you two. What good does it bring if she asks for her EX's FUC*ING opinion?! Nothing too unbiased. What does she expect out of this relationship in the future and the ex's involvement in her life? Tell her how you feel and ask her how would she feel if you talked to your ex about everything. Reverse the roles, she'd feel like sh*t too.