Whats a good excuse to not hang out with my friend today?

So he asked if I wanted to hang out yesterday on Facebook. I said sure, but theres this one kid who always tags along, I cant stand him. I also wanted to hang out with this one girl tomorrow. The other problem is, my friend is like a big brother to this girl.... It's like a giant cluster ****. I need a good excuse to get out of hanging out with him today.