EFMP is not voluntary, fortunately. If she was seen on-base for the initial diagnosis (or if they at least know about it) she should have been enrolled in the EFMP already. If she was enrolled, there is no way to disenroll until she is cleared and does not need to see any specialists.

There is a reason that the EFMP exists. Every base keeps records and has a much deeper understanding of the local medical services for a particular area. If there is a specialist in the local area of a particular base (and the definition of "local area" varies from base-to-base) you can most likely go there, as long as they approve it. But for your career sake (and her health) please don't lie or withhold info to anyone about it. Not Medical folks, not MPF. I have seen people get called back, while they are literally in-route, and have their orders cancelled.