My Muslim best friend turned Atheist, what am I supposed to do?

Pretty much the title. My Muslim (now ex) college best friend recently turned Atheist and I'm not sure what I should do now. I've tried my best to convince him back to Islam but now after weeks of arguments I'm almost sure he won't be coming back. He says he's only told me and one other friend about this yet. I've heard quite a few times that the penalty of leaving Islam willingly is death so I'm not sure what I should do and who I should tell because after all he's my best friend. I'm afraid if I too will be held responsible if I decide to not do anything. Thanks!