It's a dumb argument.
People aren't just their gender. That's a big part of them, but there's so many individual things about anyone that you can relate to.
You can relate to everything about a character, the experiences they have. You can relate to the people they know, and how they interact with them. You can relate to the character's past, and their emotions. You can relate to a character who's feeling scared, alone, nervous, curious, brave. All of these things are traits that make them relatable. It's up to you what you identify with. Gender plays a part in some of this, but by no means is in invalidating any of that.
Unless a game is solely about topics and experiences exclusive to a specific gender or race, like about a girls adventures through her first menstruation, then I don't buy the issue with being unable to relate to a character because of their gender. It just doesn't make sense. Especially in your friends case, wherein Horizon has almost nothing to do with the fact Aloy is a woman, unless you want to talk about some of the themes involving motherhood/daughters that it explores. But even then, if you have a mother you can relate a LITTLE bit.