If passport says female

I do genuinely sympathise, but you aren't responsible for bringing this person through this situation. It's your friend's child and your role is mainly limited to supporting your friend while she supports her child. Both you and your friend need to have a solid understanding that compassion and support do not require believing the fantasies of someone who isn't mentally stable. You can be grounded in factual reality whilst still helping someone who desperately wants you to believe something that isn't true. A life spend hiding from the real world will not be happy.

Under the Equality Act everyone has the right to equal treatment and with the protected characteristic of Gender Reassignment this young person has legal protection from every kind of illegal discrimination. A fundamental principle under the act is that generally the default is that all services and opportunities are open to everyone so the vast majority of things have no barriers to them. Where there is a legitimate reason for something to be single-sex, that's allowed, and males who identify as women do not get access to female single-sex things because their sex is still male. If they aren't rational enough to accept that then they aren't ready to leave the house yet.

If in the UK then no, he can't. He remains a man for all legal purposes, regardless of passport, driving licence, GRC.... The whole point of the FWS Supreme Court case was to decide whether transpeople with a GRC should be treated as the opposite sex in respect of targets for gender representation on public boards in Scotland. The conclusion was that no, they shouldn't, and "woman" in law describes biological women only, not men who want to be women. Self-ID was never the law in this country, and the Supreme Court case affirmed that a GRC did not change the meaning of "woman" either.

The ruling extends beyond board representation, and applies to toilets, changing rooms, sports, jobs advertised as women-only, and numerous other things. The fact that some transwomen ignore the law and use women's provision anyway doesn't change this.

Changing the passport to say F just indicates the gender that the person identifies as, that's all. A assport saying F does not prove anything.

When services are provided for males and females separately, that is on the basis of sex, which does not change, and a male person with a female gender and a passport saying F is still make sex. It's generally accepted by most people that if it would cause a male person distress to ha e to use the mens loos then that's an accessibility need and it's perfectly reasonable touse the unisex accessible loo which is there for the needs of a very wide range if accessibility needs, not all of which are physical needs and they are not just for people with mobility issues.

Sex and gender are two different things. Gender is controlled by the individual. Sex cannot be changed and for this individual will always be male. There is no hormone level, surgical procedure or official document that can change that. Making peace with that, whilst also living as liberated from the chains of gender and expressing femininity in clothes, hair, mannerisms, name etc, is the combination that will be healthiest and lead to the greatest achievable contentment

Single sex services and spaces are - as per the FWS Supreme Court case of April last year - about biology.

Passing, paperwork, and internal feelings are all irrelevant.