What a confusing thread.
OP, can you come back on and provide a bit more detail about what you're looking for from it?
Are you asking for advice on how to support your son and his male-identifying, biologically female girlfriend?
Was the "performative feminity" conversation with your son or his girlfriend? I assume your son.
(Does he use the word "girlfriend" to describe his partner? Presumably not but it's helpful to know if you're looking for advice).
And yes, there are as many ways to "be" a woman as there are ways to "be" a human being. The only requirement for the former is to be a biologically female adult. Nobody can change their biological sex, even if they believe themselves to have a gender identity that differs from it. The only requirement for the latter is to be a living homo sapien. Nobody can change their species, even if they believe they have a furry, therian or otherkin identity that differs from it.
"Being" a woman (or a human), beyond the actual fact of being one, is simply about living your life. It's all down to a combination of personality, learned behaviours, circumstance and choice. "Performative feminity" and "performative masculinity" are a part of that mix, based on stereotypical behaviours of either sex. Most people adopt at least some stereotypical behaviours relating to their sex - that's why they are stereotypical.
I could be a woman (and a human) in the UK or a woman (and a human) in Afghanistan. My biological sex and species remains the same regardless of which clothes I wear, whether I take cross-sex hormones, chop off body parts, "perform masculinity" or "perform femininity" (or walk around on all fours). However, in each of those different locations my way of "being a woman" (the stereotypical, culturally influenced, sex-based behaviours and styles I adopt or reject as a female human) will differ greatly i.e. my experiences of being a woman will differ because women in Afghanistan don't have anywhere near the same freedom of choice as we do in the UK.
Anyway, without context about what you're looking for from this thread OP, it's difficult to add further comment.
Edited for typo.