Is my friend a good friend?

I use to be best friends with this one girl named *Lisa, and we both had a mutaual friend named*Belle. Me and Belle stopped being friends with Lisa cause she was a liar and a backstabber. So I do not speak to Lisa at all, we both hate each other. And now me and Belle are best friends. And Belle talks a lot of **** about Lisa with me. But when Lisa asks her to hang out or something, and Belles with me, Belle lies about it and says she has to be with her family or something, it hurts that she would do that. I dont know why she can;t just tell her, is she being a good friend or what?