Yes to both questions.
played CS:GO very well with HD6770, and your HD6850 is stronger than HD6770 , so it is confirmed that it will work well.
Now about rocket league - About settings, well, I am not sure on which but it will be playable.
And for 30$ I think this is very good deal.
And last thing is that I hope that your friend has OK processor=CPU so he can play above games.
Good luck