I would say the first step is to ask your friend what their prefered pronoun is. If they prefer the pronouns he/him and you respect them, try to start using those pronouns instead. It would also be a good idea to know how they feel about their gender. Ask them if they are ok with being called female or if it makes them uncomfortable. I'm assuming by the use of a 'man's mentality' this person doesn't identify with their gender assigned to them at birth - in which case it sounds like they may be transgender. If they were a tomboy they very likely wouldn't be interested in changing their pronouns. It is perfectly ok to use they/them pronouns as well and it's a popular thing for people who identify as gender neutral or non-binary like me. I think the biggest thing is being respectful to your friend and using what makes them comfortable. I'm open for more discussion should you want my advice at all. Please also remember that gender is a confusing thing and your friend may be trying to figure out exactly how they feel and therefore their gender identification could fluctuate.

In the UK especially in the larger towns and cities that are used to many people and different types of people migrating there. If you just introduce your friend by name. hello may I introduce you to my friend ( ). people will very quickly work things out for themselves and nothing will be said. you could be describing a "tomboy" woman who likes doing male things during teenage years often they grow out of this phase.

And no, "that guy" is not neutral, it is a masculine reference. The feminine would be "that girl" or "that lady" or "that woman".

I agree that in this situation it is best to ask the person which pronoun they prefer. In Canada, in the transgender community it is becoming more common to use the pronoun "they" even in the singular: "Alex went to work today, they were running late though." This is gaining more acceptance.

Thank you so much! I've never thought up "they/them" can use in singular. This friend is a very bright person. They never hidden, they says "I'm woman biologically, but I live as I want. So what's wrong?" I'm proud of them. I imagine if I ask them about it, maybe they laughs "I don't care. Such a small thing." I know they're struggling. I will keep being respectful to them. Thank you, again. I got more than English knowledge.