Pay attention to how your friends treat other people, because how they treat people outside of their inner circle says a lot about their character.

Personally, I wouldn't want a close friend who is nice to me yet shows unwarranted rudeness to others.

As harsh as this may seem; some people have terrible friends. Those friends may not be terrible to them directly, but those friends are terrible to others - demonstrated via bullying, rudeness, or even abuse...

Pay attention to how your friends treat others, and when they treat others badly; check them. A friend you can't be honest with is a friend you have to be fake around, and that's not a healthy friendship.

If your friend refuses to accept constructive criticism, and continues to demonstrate negative behaviours towards others, just know that those negative behaviours will soon impact you directly. Also, ask yourself whether that kind of 'friend' is the kind of friend you want.