No it is not good to steal a mates g/f, but he started the whole thing off and didn`t care he did it to you, And as he stuffed up with this new girl and she was upset mean`t she didn`t want your mate, So you are not stealing off him as he did to you, So go for it and enjoy yourself.
Years ago I had a mates current g/f ask me to take her out, But as said to her ,No as i don`t take my mates girl. As i am true mate to the friends i have. And it should not be done.
But if he had dropped her and she asked then i would have picked her up as he didn`t want her
But best to pick up girls on your own as mates can get in the way, and you lose the girl to them.