Tandora
The person in front of you is not a lie. They are real; their experience is real.
Just for the record, I did not at any point suggest that any person "was" a lie, and I do not suggest this.
Delusional, yes. It is a delusion to believe that you are of a different sex, sexual orientation, race, or colour than the one you actually are.
It is often painful to have to face the truth about things. Having to face the fact that you are colour-blind or tone-deaf or under five foot tall or a woman with size ten feet is quite likely to be painful. But it absolutely does not help to be lyingly told you can tell red from green or have perfect pitch or are five foot eight or have size six feet. Someone lying about any of these things does not make them so, and you can wish until you are blue in the face and they will still not be true.
Tandora
All you need to do is to hear them, to listen, and to understand that their experience is real .
I do not dispute that their experience is their subjective reality; unfortunately, since magic does not exist, I am unable to change anybody's sex for him or her, nor alter how he or she feels about the sex that he or she is, so it remains entirely subjective, and that male person remains objectively male, that female person remains objectively female. I am happy for them to continue in their delusion, holding by the maxim "do what though wilt shall be the whole of the law, so long as thou harmest none." Unfortunately there are rather too many delusional men whose beliefs do harm others. I do not regard their feelings as being more important than any other person's feelings.
I understand that there are medical professionals in the field of mental health who may be able to help people come to terms with their delusions and live with them, or even to recognise them as being delusions, but I am not one of those. And I definitely do not think that chemical or surgical intervention to alter the bodies of those with mental problems is a good idea.