Most Helpful Guy
Most Helpful Girl
You are NOT wrong for saying no. Please don't feel bad. You are not obligated to take them grocery shopping. I know why you feel bad, you want to help them and his sister didn't do anything. However, it's too risky to go there and possibly see that guy.
TBH you are far better off without him. Just do you, go out with friends, have a good time, enjoy your life. You deserve it! Pamper yourself! It's okay to say no if there is a possibility you are going to be in a situation where you will see this guy and possibly the new girl.
I can't believe he called you to tell you about how great his new girl is. WHO EVEN DOES THAT? Sounds to me like he is trying compensating for losing you. You were probably really good to him, by the sounds of it you were.
Just be glad that you no longer have to fund his sorry butt. Spend that money on yourself :) Get your hair done, nails done, some new clothes, or even just a movie with friends. Do something you enjoy :) Don't feel bad!
If I were in your shoes, I would have said the same thing! You did nothing wrong!