"While I don't pretend to be an expert, I do find the different arguments and evidence that trans women are somehow a danger to other women totally unconvincing. Like all bad arguments, they take one or two rare incidents, and try to hang a full-blown thesis on them. These are not "legitimate concerns' and I don't believe we should engage with them."
That's part of the problem, right there.
Whether or not transwomen can be a danger to women isn't an "argument", a matter of belief. It's either true or it isn't. That's what he should engage with, not dismiss them as "not legitimate concerns".
The statistics strongly suggest that transwomen are more of a danger, especially if they still have male genitalia, in which case they are capable of raping us. The statistics of the rates of sex offending among trans prisoners show this. It's not just "one or two rare incidents".
Not all transwomen are rapists, of course, just as not all men are rapists. But all rapists are men or transwomen, and that's why we want safe spaces that keep them out.