I cannot imagine that a man with breast cancer and a woman with breast cancer are having the same kind of psychological experience.
The physical experience will have some commonalities, sure, but the rest of it? Nope.
Plus, we are all aware of how a single male entering a group of female people can completely change the mood and energy. Women use groups like this to be frank with other women about how they feel about their changed bodies, to discuss really difficult and personal stuff like breast reconstruction options.
Facebook support groups aren’t really about the medical aspects (it would be irresponsible if they were, seeing as the posters are not qualified) but about day to day living with a disease or disability.
Sure, he could’ve been rejected more kindly, but surely the solution is to start a group specifically for men with breast cancer, or to join forces with a group for men with a different type of cancer (testicle, prostate etc) and be a subset within it.
There are plenty of online support forums where the posters identity is largely anon, like here, where this chap could read and post about the bits of having breast cancer that are relevant. Quite a few of the major cancer charities have online forums.