You should have asked your friends why a black person can't be racist, because to most white people it's an absurd proposition. So allow me to explain a bit here.
Racism, unlike what we mostly think, is not a state of mind or an emotion. Racism is a sociological system that transcends the intentions of any one individual. In case US history is not forefront in anyone's mind: 150 years ago, a black person had the same legal status as a pig; 100 years ago, it was illegal for a white person to marry a black person; 60 years ago, it was illegal for a black person to drink at a water fountain.
Today, the likelihood that you'll be shot by a cop if you're black is astronomically higher than if you're white. If you have a black sounding name, you're less likely to have a company respond to your job application.
These things are institutional. When we say a black person can't be racist -- we don't mean they can't be prejudiced. They can, just like all people. But the privileges and paved roads that a white person enjoys simply aren't there by default for a POC, so they can't benefit from institutional racism.
TL;DR - false equivalency. "Black X Thing" is in the context of a historically disenfranchised people overcoming an entire country hellbent on keeping them disenfranchised. "White X Thing" is ridiculous because it's redundant: "Thing" is already White by default.