I've had experience on both ends, and I got very frustrated when the seller refused to look into the situation. I never did receive the item, because the tracking said it was received and they wouldn't replace it - that's something that makes me very nervous if people buy tracked shipping, or if I receive tracked shipping. It doesn't give me peace of mind at all! It just makes me worry!
I'd contact the courier, ask the buyer to check with their local postal depot and with their neighbours, and anywhere that it could have been left by the postman. Tell her to give it a week, and if it's not arrived, then I would replace it. It may have been marked as delivered sooner than it actually has been, which I've seen happen twice, and it may well turn up in a day or two.