This is one tough area to navigate. Be cautious and very certain of where you stand in the issue.
I'd be unlikely to report to corporate because you will be caught in it and if there is badness there or not - there is no upside for you. CMS has a direct contact on their webpage for FRAUD reporting - but I'd get an attorney to advise me BEFORE I do anything.
Any organization will do WHATEVER it takes to defend itself and S&!$ rolls downhill - so, be SURE that you can get caught in it.
CMS rules are complicated and just because a provider is "doing something" questionable may not mean they are billing for it in a way that may violate standards as they relate to money.
Clinical standards vs. reimbursement practices can get tricky. I've seen some wicked questionable clinical decisions and practices that DO NOT translate to medicare fraud. And I have seen other cases where "stuff" is being done in a questionable manner and those employed just go along, because they it is "how things are done". There are any number of tough spots in between. Example: A provider provides a clinical service and either manipulates the clinical "presentation" or pencil-whips the record to make it look a more complex case than it is (or was) and then bills CMS requesting a rate of reimbursement not truly supported clinically - medicare fraud perhaps, but really hard to report without the one reporting having enough intimate knowledge to make the reporter's involvement questionable.
If this is, in fact, related to money matters and you can divulge WITHOUT violating any confidential info - proceed with caution. I'd hate to see good intentions take you down...
If you can proceed and "want to eat lunch in this town again" (keep any chance of working - for anyone) I'd think it wise to retain experienced counsel with experience in CMS regulations and "whistleblower" protections. If you report fraud and they find it - there is an economic "incentive" for the one that reports it to be reimbursed a % of what they (CMS) recovers --- so, an attorney may be willing to take the case on that term.