Is the proper term for saying none 'mil' or 'nil'?

Hi, thank you for your help, this has been bugging me for quite a while.

So I have a friend who is on my sports team and is from England (we live in the US) and whenever she is talking about how we beat a team without letting them get any points on us (ie. 4-0), she goes "yeah, we won four-mil."

Recently on TV I saw a sports station interviewing a player (American) on how his team won a game with no points being scored on his team, and he went "nil" instead of "mil". I asked my parents which was the proper term and they said "mil" was incorrect.

But what bugged me was both the girl's parents said "mil" (also had strong British accents) and my coach for our sports team (who was British as well). They say it all the time, so I know I am not mishearing them, too.

So google hasn't been much of a help on this (or maybe I'm not a very good searcher...), but I was wondering if anyone knows the correct term.

Which brings me to my question: Is the proper term for saying none 'mil' or 'nil'?