What is a true atheist?

I was talking to this 10 year old atheist kid i know and my friend who is now a Christian said she used to be atheist. She told the kid she turned atheist after her grandma got cancer and died. Then he said yeah but you were never a true atheist you were just mad at god. He said true atheist don't believe in god at all usually because of lack of evidence. But isn't that just like when Christians tell atheists they were never true Christians? If not explain why thanks