To answer your first question, even though this opportunity has passed, in the future you can gauge how much anxiety would be caused by hanging out with him; if it's causing you too much anxiety, you can be polite and say that you can't (like your other friend can), and can continue working on the therapy and the strategies from it. If you're finding that the idea of hanging out with this friend is causing too much anxiety, you can always work your way up to hanging out with them, by hanging out with friends who you're more comfortable hanging out with, who don't cause you too much anxiety. Since you are still going thru the therapy and haven't completed it yet, you shouldn't worry or stress too much over things like this that cause too much anxiety; just keep doing what you're doing, progressing through the series, and keep gently moving forward and doing things that cause you some anxiety, but not too much anxiety. The best advice I can give is keep going with the therapy and working on moving up your hierarchy, and eventually your brain will figure things out and you'll find yourself doing things such as hanging out with friends over a drink.
Hope that helps.