Boy asks girl to marry him over the internet

Assalamu Alaikum Respected Mufti. A young baaligh girl befriended a boy over the Internet and the boy asked if she would marry him, she agreed. The girl heard that if two people agree to marry than nikaah takes place and so she considered them married but never met with the boy, it was only chatting over the phone. there were no witnesses at the time the girl agreed to marry the boy but there were 2 or 3 others who read the chats, this was NOT done with an intention of witnessing but rather casual interest. P.s. the boy and girl mentioned in the chat that we are now married.
Has nikaah taken place?
Jazakallahu Khair