"You are always welcome, and I love you, but you don't get to decide who is in my circle."

I wouldn’t be best friends with someone who had that attitude

I'd tell them this isn't childhood and I can pick my own friends.

I would be running dry on best friends for someone giving me an ultimatum like that.
I demand respect first and that is crawling over all types of personal boundaries.
The only way that is cool would be if they knew of assaults, murders or drug pushing I wasn’t aware of.

I'd ask, "what are we, 10 yr olds?" That's so ridiculous. I choose to have friends...plural.

Bye

I would try to make her see how needy and inappropriate ultimatums are. If she still feels the same way it would be the end

I’d want to know why my best friend said that. What’s their reason for giving an ultimatum like this?

I'm in a similar situation rn T_T I've been keeping the new friend a secret

Considering how long and how well I know my best friend, it would have to be for a serious reason. She would never make such a request otherwise.

Old friend if they have been a best friend to me

Whatever you're both imaginary

Goodbye...

That’s so childish. I would never agree to that

I'd have them both fight to the death