I completely understand your concerns. Eating rocks or any other sort of foreign object is a good way for them to catch a disease/parasite, or chip their teeth, but most concerning, it is a good way to get an abdominal obstruction which many times requires surgery.
Because of that, I will be very honest with you. Getting the pup to curb this habit is going to be a long tricky process that will require a lot of patience and training on your (and the owners) part. First and foremost, you must never leave her outside where they can reach the rocks unsupervised, and you must always be watchful while on walks. With that, it is important that you always have treats/toys or whatever the dog enjoys most. Every time she reaches for a rock, you will give your command (drop/leave/watch or whatever you would like) for them to leave the rock alone and instead focus on you. When they do this successfully, give them their prize and lots of praise. The key part, you must do this every single time. Puppies are very easy to train if you are doing it correctly, and most of them are easily trained away from bad habits with nothing but love and treats!
I know its tedious, but it is so important that you curb her behavior before it becomes permanent.
Best of luck!