First of all, I don't believe that it violates copyright in any way to loan a pattern to a friend, unless you know or strongly suspect that she will be copying the pattern.
Is your concern that this lady won't return the pattern? Or that she will copy it? Or just that you don't want to share what you have purchased with someone who is clearly wanting a freebie? Any of these are legitimate concerns, but you need to be clear with yourself about what is your cause for concern.
I would not lie, not make excuses, just tell her that you don't loan patterns because (you are afraid of not getting them back; you do not want them copied because of copyright concerns; you paid for it with good money and you expect others should do the same). Pick your response and stick to it. It is not something to be ashamed of, but something that is true and worth protecting.