There are many types of Health savings accounts that have no requirements and don't bother to coordinate benefits at all, so it is possible to use that rather than the insurance. My own HSA only requires me to submit receipts and I can use it for anything health related, even over the counter meds.
But, none the less your thinking is correct, she has no benefit to NOT filing her insurance. YES patients may opt to not have claims filed even if the provider is participating but as you mentioned, she won't have any of her money applied to deductible.
Another thing to consider is that if she's going to be receiving a cash pay discount, there is a good chance she may end up submitting claims herself to have the actual CHARGE applied to her deductible..be careful with that and if you do offer her a discount, make sure that the itemized billing receipt/charge slip SHOWS the actual charge, the discount and her payment in case she should submit it. If you only provide her a bill showing the discount amount as the charge and she submits it, it would be considered a false claim on your end as well as hers!