Can white people be racist to other white people?

My friend is convinced that this cannot happen. But I know a Polish guy that mocks russians and makes stereotypes at them, I told my friend this and she said that this wasn't racism and I would humiliate myself if I was to say this to other people. Who is right???
Update: @ <3 I know, I am half Serbian
Update 2: they are horrible to each other
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