He's a man. He has no idea how girls and women navigate a male dominated world or the way male violence and sexual aggression affect us.
Tallulahwaitingintherain made a brilliant comment yesterday. Maybe this will help your friend understand. I haven't bolded it, but here it is:
Lets do a thought experiment. Let's imagine there's another category of human. They're substantially bigger and stronger than men and immediately distinguishable from you due to an exaggerated brow ridge, pointy shoulders and other physical indications specific to them. They are noticeably more aggressive than men and commit the vast majority of violent crime, much of it, especially sex offences, against men. The majority are sexually interested in men and a substantial minority make it clear that because of this they think of men as a kind of prey. They are politically powerful, better paid than men and pride themselves on being as different from you as possible.
Since you were about 10 years old, you have been regularly sexually harassed and humiliated by these people: at school, at work, on public transport, on the streets. You don't argue back because they're often aggressive, you know they're a lot stronger than you, and because the whole point is that they are getting off on exerting control over you and you are keen to avoid escalating their aggression. You and most men you know have been seriously sexually assaulted by at least one of them on at least one occasion, and you have been in many, many situations where you have been frightened this was going to happen. When you turn on the news, you see reports about men being raped or murdered by them, and when you watch a film or tv drama there's typically an obligatory rape scene in which a man is their victim. You know that many of them enjoy watching porn in which men are violently sexually degraded and you are aware that some of them get off on invading men's privacy by watching or filming you in toilets and changing rooms. You also know that most institutions are run by these people and that if you are harmed by one of them you will have trouble getting anyone to believe or support you. Of course, many of them are perfectly safe and reasonable but there is no way for you to tell who is a risk to you and who isn't.
Do you want these hypothetical people sharing your hospital ward or cell in prison? And can you see why women are concerned about allowing male people in our spaces even though we are completely unfussed about sharing with women who have a different skin colour, cultural background or sexual orientation than ourselves?