I would just not respond back because it would kind of give you a link back to your ex. I'm not sure if you want to get back with your ex or not. Based on your post, I would have to say that you just want to forget about her the best that you can. I mean, I guess you could respond, but I wouldn't do anything beyond that. Don't hangout with him. It's like not hanging out with your ex's friends, ya know? Relationships suck especially when they are over. There's no easy away about it. I definitely feel for you. It's pretty much up to whether or not you want to respond, but I am going to say that if you respond, he'll probably end up mentioning your ex and tell you that she still thinks about you blalblablalbla. Ya know? And that will just create a whole other mess. Lol, my ex still talks to my friends, right? Asking how I am doing and stuff. And they tell me. I'm just like yeah, okay, that's nice.. moving along. It's hard but the point of a break up is to sever the tie, and it's really up to you whether you want to get her back or not. Ask yourself questions like "is it really worth it?" I can also tell you why you are still thinking about her. You are thinking about her because the fact that she broke up with you makes you want her more. It's actually putting her on the pedastool. It's up to you man, best of luck! I know it's a difficult mess.
I will do my best to reply with an educated, humble and honest answer. Ultimately, it is up to you whether you want to listen to my advice or ignore it completely. Sometimes, my advice may be wrong; occasionally, it will be right. Regardless, I want to do my best to give people answers they are seeking.