so i broke up with this girl almost a year ago now...recently i asked out one of her good friends for a friendly coffee chat. she said yes. then the day before the coffee, my ex messages me on facebook and asks me why i invited her friend to coffee, then i said i just wanted to go for a friendly chat with her, and said thats someone normal for me to do with girls, and then i asked is there a problem, and she said no problem. ... so then today suddenly the friend flakes out on my at the last minute saying something came up blablblalba. so i replied, its okay no problem. im pretty sure she flaked out cuz my ex cockblocked me....my question is what do i say to the good friend that i want to go for coffee with now, how do i convince her to just go for a harmless coffee despite the cockblocking from my ex??

Your ex probably has a lot more influence on her than you.

However what you don't want to do is to make it seem like you are bitter that she didn't go for coffee with you. If you really want to get her make it as late notice as you can like the next day. This way it is less likely your ex can influence her.