It’s your ex. What do you care? If you and your ex’s friend are getting together, and you hit it off, it your date going to be loyal to you, or to the ex? The ex has nothing to say about it. What’s the big deal?
I had a relationship with one of my ex’s best friends. It was fine. Sometimes my ex would show up, but that was no big deal. Sometimes she want ex-sex, though I don’t really understand why. Once the three of us went on a camping trip together.
Even if you have a bad relationship with your ex, it seems to me that your new partner would not bring the ex around, or make it difficult for you. Honestly, I don’t see why this is even a question.