I have been in a similar situation years ago, regarding my oldest son, my niece and my sister. My niece was "tattling" on my ds for something, ds (who is a very honest kid - we talk about things openly and he doesn't get "in trouble" for things) denied it. My sister was accusing me of letting him get away with it (throwing something at niece - neither my sister nor I was in the room). After arguing for a few minutes with my sister on this, I said "we are just going to have to agree to disagree on this one". And we left it at that. After a few minutes of grumpy silence, we each got over it.
Sisters can generally get over things that maybe friends wouldn't though. But, I think you also have to ask yourself that if your son did say what your friend accuses him of, what would you do? I'm not saying you should do anything. I agree that you shouldn't. I think you should believe your ds on this one. But, even if he said something and you heard him, what would you do? Is it even worth discussing it with your friend? Maybe just tell her "I'll handle it" and leave it at that. It's none of her business what you do to "handle it" and then thank your ds for being honest with you.