Was George a good friend to Lennie?
This is a debatable question, but in my view, George was a good friend to Lennie for two reasons.
First, George stuck with Lennie. This counts for something. Lennie is mentally challenged. It is far easier to take care of yourself. But George took the harder path by also taking care of another person. In many ways, Lennie is like a child. Even when they are on the ranch, George gave it his best effort to take care of Lennie.
Second, when George found out that Lennie accidentally killed Curley's wife, he knew what he had to do. He knew that they had to find him before the other men. The other men would have certainly killed Lennie in a brutal way, as passions were high. So, when George found Lennie, he gave him a quick death. But even as he took his life, he left Lennie with their dream.
We might not agree with what George did, but in George's eyes he did what he thought was best. He was being a good friend.