Twitch banned because friend said a racist word?

Can I be banned if my friend says something racist in my Twitch Stream?

Yes you can. In principle, you are responsible for everything that happens in your stream.

However, the likelihood that you will actually be banned is fairly low, as Twitch is more likely to be made aware of the larger streamers. However, I would make sure that something like this does not happen again because your viewers can still report you.