Okay, I was going to recommend something like this, but If you got it handled already, never mind
Es tut uns leid, wenn Sie nahm es der falsche Weg, aber ich bin nur ein Freund, mehr nicht - Sorry if you took it the wrong way, but I'm just a friend, nothing more
By using the Sie formal you are signifying that she is not someone close to you where as du is informal and signifies a friendship or relation of sorts