Will being diagnosed with PCOS result in a medical board?

No. The authority for medical retention is AR 40-501, and DoDI 6130.03 Volume 2. PCOS is generally well managed with changes to diet and sometimes medications. Sometimes the medications may limit deployability and/or require waivers, but there is no specific requirement to refer PCOS for an MEB.

I don't think so but, as stated, that is something she needs to talk to her PCM about. Her PCM would be the one to initiate it.

I just did a quick Google search and found this: https://academic.oup.com/milmed/article/184/9-10/440/5366285

It may be useful for you both to read maybe? I don't know if it causes a medboard but when some things PCOS cause are overweight/obesity and menstrual irregularities, that could trigger a medboard if it can't be controlled. This seems to say that active duty women don't really even get diagnosed properly with PCOS.

This came up too: https://clinicaltrials.gov/ct2/show/NCT03876665 - It's titled "The Experience of Living With Polycystic Ovary Syndrome in the Military (PCOS)."

And this; "Researchers say the causes of PCOS are complicated, but insulin resistance and hormone regulation are key factors. You may be able to manage these factors and ease your symptoms through lifestyle changes and dietary supplements, but there's no one-size-fits-all approach to treatment."

It seems like if she and her PCM can find something to help her manage it she may be fine. I have no personal experience with PCOS - so it's safe to say she needs to talk to her PCM to answer all her questions. If necessary, her PCM could refer her to any specialists that deal with this too.

The best information will be from her Doctor. The Doctor will be the one who initiates the process.