> I'm concerned about the fairness of this decision to other team members who have been with us for over a year, contributing significantly to our project. Additionally, I feel somewhat sidelined, as my friend and the new member often reach consensus without involving me in the preliminary discussions.

Literally just say that to them. You said they're a close friend, so you should be able to start off with a frank and reasonable discussion about it without beating around the bush. If you somehow can't come to an understanding, then involve a lawyer.

Honestly if I were in your position, I'd be more worried that my close friend and co-founder is already planning to marry someone that they've only been in a relationship with for less than a month. But maybe I'm just ignorant of their culture.