I assume that the insurance company is saying that your friend is faking or that he must have brought this on himself. The first is ridiculous if he has severe observable symptoms and if his doctor will attest to his health. The second...well, insurance companies treat cancer patients who are lifelong smokers, they cover ER services for drug overdoses, and they approve treatment for PWAs who admit that they contracted the virus through unprotected anal sex with a stranger. Why are trans people always exceptions to the usual policy?
I have never seen an insurance exclusion for cancer, drug overdoses, or AIDS. Go figure.
Flan is right. The physician has to code the condition properly. But I wonder what other factors are at work here. Is your friend listed as male or as female with the insurance company? I'm assuming female. If he is identified as male, that's a whole different issue, I think. I assume that his insurance provider knows he is trans, and that's why they are denying him. Is he on testosterone? Is this a trans issue?
If it is clearly a trans issue, and if working with the doctors and appropriate coding doesn't work, I suggest that your friend google trans law sites (like the Transgender Law Center, which operates in California) and start looking into patient advocacy groups. There is also a trans health conference that has been running for a couple of years--their site might provide some useful references.
http://conference.transadvocacy.com/.
He probably needs to make a big stink about this. I'm glad you're on his side. We need more folks like you.