I've read every post in this thread. I think almost everyone here is being way too dramatic.
OP, it was good and decent of you to agree to this. Don't listen to these other people. He is leaving for good and will likely not be back. You will likely never see him again. Despite the fact you guys are done, you were important in his life, and he was a significant person in your life. Life is short. Meeting one last time just to chat and say goodbye is not a big deal and is the least you can do, especially if he's buying. Given you are not crazy about the idea, lunch is a great idea as it's more casual and generally quicker. If he tries to get sentimental, be kind, but don't allow too much of it. If it gets awkward, just remind yourself you're never going to see him after this, so hopefully you can endure 5-20min of awkwardness.
Others, you guys are being either Aholes or idiots. Do not encourage girls to do things for guys out of pity. That's pathetic. And guys, NEVER attempt to get a girl's attention through pity. It makes you look terrible and makes the girl uneasy. This girl should go and see the guy just out of common decency. I broke up with my last girlfriend a year ago and have no desire to ever speak to her again (not that she was a bad person; we just weren't right for each other), but if she asked me to meet with her I would oblige. Meeting with someone a single time is not asking much, especially given the circumstances.
Please do let us know how it goes.