He was talking with the LORD wasn't he?
That is what Prophets do. They hear from the LORD and tell someone what the LORD has said. And Adam, the man, told the woman that the LORD said, that if they ate of that tree of knowledge, they would die.
Being a Prophet doesn't mean that someone is leading anyone. It means that the Prophet heard the LORD say something and tells others what the LORD has said.
And I am sure that the woman heard the LORD too. So she was a Prophetess.