Would you be upset? Explain your answers, please!

This afternoon I got a message from my best friend's ex/baby daddy.

He told me that my ex-husband and my best friend have been going out for lunch (with her daughter) together and telling her daughter to lie to him about it so I wouldn't find out.

Of course, I asked them both if it was true.

My friend said she'd call me later (I texted her the screenshot of his message) and my ex-husband basically just told me yes, but it's none of my business because they're just friends and he's allowed to have friends.

After that, I sent a message to my friend telling her how hurt I was because I never thought twice to stop talking to any of her exes because she's my best friend.

I have told both of them in the past that I wasn't okay with them being friends. To me, it's disrespectful and inappropriate.

Would you be mad? Betrayed?