The sad part about all this is that by presenting and alternative narrative to his own in good light with a question as to how the facts presented actually play out, we might have had a good discussion.
For example, even if it's true that on average women do worse in general in technical or engineering fields, that doesn't necessarily mean that the women in those fields currently are worse on average. There are a few possible reasons why they may be better than the average men in those fields.
Firstly, if we accept that women have different levels of aptitude in certain skill sets, there's no reason to assume they have the same variability. It's entirely possible that women on average are worse in certain skills, but that they have more exceptional outliers.
Secondly, even if there is not more variability in the female population for the skills in question, it's entirely possible that cultural and social pressure has resulted in those that choose careers on those skills having higher than average abilities in them. For example, if my family has a legacy of being fairly clumsy and uncoordinated, but my brother is a pro athlete, what's more likely, that he's a crappy athlete, or that perhaps the criticisms that apply well to my family do not apply accurately to him, for whatever reason?
Providing an olive branch can make all the difference in a discussion like this, and the fact that neither explanation put forth here is contradictory with each other or the premise that perhaps a 50/50 gender split isn't necessarily ideal makes this particularly lamentable, since we might have been able to have a good discussion about gender differences while minimizing the impact it has on individual members of that gender. Of both sides (there's plenty of fields where men face an uphill battle too).