I haven't seen or talked to my Ex in over 5-6 months now. We dated for 2 years. I am doing the no contact thing (even though at first I didn't) and it is really hard. I know she is with someone else but am slowly starting to not care. Anyway, I had a huge costume/birthday party planned for about 6 months for my 30th birthday. My ex decided to call my best friend the day of my party and leave him a random voice mail even though he has cut off contact with her also. My best friend ended up responding to her via text message. He told me he just said that he had to go to my party and that he couldn't talk and told her not to respond because his phone would be off. My question is: Do I have the right to be a little upset at my friend for even responding to her? I don't want her in my life anymore and expect that "my" best friend of 15 years should respect my feelings.

He told her not to respond to him, I don't really see an issue. I mean, if he said "let's meet up" then yea, be mad. But don't sweat this

I think your stressing over nothing. Like Rome says he told her not to respond.

Get over it, that's petty.

Get over it, thats petty.
I got to say I agree. No harm done. If more happens then maybe you can be upset. Perhaps try talking to your friend about it?

I don't think he said or did anything worth getting upset over. He actually kind of cut her off if you ask me. If you think about it, he didn't have to even tell you anything about it. He could have secretly communicated with her.