C.W.
I don't get why your mad at her. Did she have a bad attitude? From the written words she sounded very sympathetic, like she understood he may not want to be there and didn't want to force him there. Honestly, I would encourage him to talk to her (encourage him in a non-confrontational way) but don't force it. I would not force my child to see them (her dad's mom has seen her once, because of money issues, and her dad's father has never met her). I would encourage him to see them and maybe meet up in a park so it is a stress-free environment. I'm sure she would compromise if she wants to see him. Have a casual talk with him about why he is scared of going over there. Why does he not want to go? I know you say it's because of the crowd but still talk to him about it so you can subside his fears and he will feel like you understand and that trust will be there the next time you go. The next few visits for quite a while should not be more than a few hours. Expecting him to visit for hours and hours with adults he doesn't really know probably won't work.
Update: Oh Wow! Maybe you need to have a talk with his grandmother about her expectations. She can't just expect him to want to be over there all the time and be her best friend when they never see each other. Maybe having a talk about how he is nervous to be around her because of how she came off last time is in order. If she doesn't understand and compromise on stress-free environments, then he doesn't need people that are going to make him uncomfortable around. I'd be mad at someone too if they had a nasty attitude with my child. As for you, maybe in time you will get less angry at her for it, I would be upset too. Can you talk to her about what happened without it turning into a fight?