When you see the discussion is moving from a discussion to an argument, better leave it before it gets too hot.
You may try change the topic to some topic that she likes and you don't know anything/much about it. This way you will be just accepting & agreeing to what she is saying; and she would be happy in the end of conversation/discussion. And you will also not have to compromise on your view point on the initial topic.
"A discussion is when you want to know what is right, an argument is when you want to know who is right."
Best Wishes. :)