I'm going to date the girl my friend used to like
why do you want to date that particular girl ?
If I were to date the girl he likes he would be highly annoyed at me.
It seems like you intentionally want to hurt your friend by dating this specific girl. You should consider how much you will damage the friendship by doing this.
I just want to troll him, I'm not serious.
Taking this action will be disrespectful to your friend's feelings. It would be best to avoid intentionally annoying your friend because you can damage your friendship.