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O.o .....
Ummmm.... Set him up on a blind date? maybe if he hears all the things involved with being a girl, he'll be less likely to want to be one. yeah! set him up with a girl w/ a psychology or medical major and have them talk about all the things girls go through just because they're girls. Maybe it'll change his mind.
wild idea, that's all.

tell him that why he can't be a girl on earth and a guy in heaven is because being a girl on earth will ruin everything.
We were placed on this earth to get a body and gain learning and experience.
Is it D&C 130: 18-19 that says if he gains more knowledge and intelegince in this life he'll have so much the advantage in the world to come. His wife should learn the girl stuff and he should learn the guy stuff so they know a lot in the end.
Wasn't there a general authority that said that behaviors and stuff will follow us also? you don't want to be in the terrestial kingdom for the rest of your life thinking "I was a girl for only a fractions of a second and now I can't think like a guy."
Just cause we die doesn't mean the slate is wiped clean and we are made pure and perfect. And even through the repentance process we must live with the consequences of our actions.

it sounds as though there is a lot of stuff going on in this person's life. You need help to help him - you can not do it all yourself. This is something that must have built up for a while for him and you can not make everything all better right away. He has to take the resposibility to question his decision and get a lot of different opinions - just as anyone having any kind of surgery (but especially life changing surgery) should.
As for having children - there are so many out there waiting to be adopted - and he would get to choose! That is just as special and important a calling as a biological parent - I would think even more so because animals can sire offspring, but it takes a certain type of person to be a good parent.
Talk to others you know you can trust to help you shoulder this burden.

What makes you think that you can say anything that will change his mind? I mean, if he's gotten to the point where he actually wants a sex change I don't think that setting him up on a blind date isn't going to change his mind. People have to make their own choices and you need to accept that. Agency. It's a great thing but it also means that people can make the wrong choice if they want to.

The aforementioned reasons are all legit, and I support them completely.

If he needs medical reasons (note: these are conjecture based on fact. see #4):
1) He would have to take hormonal supplements for the rest of his life. While this would help him acchieve certain "desireable" results, they can create a legion of unwanted complications and can, though the chances are quite low, assist the development of a debilitating and/or fatal disease. Boobs really aren't worth that. (pardon the candor razz ) Even when women, who can handle estrogen much much better than men, recieve high levels of estrogen, their bodies go haywire. The resulting issues that can develop include breast cancer (if memory serves me right) and endometriosis (hemorraging of the uterine walls), among others. Behavioral changes will ensue. Psycosis plausible due to personality spikes caused by rapid hormonal changes.
2) The surgey itself can be botched easier than one may think. And botching up that kinda job can lead to some serious issues. If things aren't connected just right, the kidneys will but unable to properly dispose of the urea and pollutants in the blood, leading to kidney failure. He would have to be placed on dialysis for the rest of his life. If such a process is not taken care of soon enough, the toxins in the blood will accumulate and overwhelm the liver, leading to liver failure. He'll know this has happened when his eyes turn the same yellow color of his skin, which will be the result of the kidney failure. Dialysis can help filter the blood, but the liver and the kidneys are now shot. Both would have to be replaced. However, due to his recent surgery, double-organ failure and debatable mental status, he will be far from the top of the donor list. He probably, and by probably I mean I'm pretty darn sure, won't even qualify to be placed on the donor list. This would be the end of your friend. It's harsh, but it is a plausible risk.
3) There are others - have him consult with a (rational) doctor (who will explain the risks completely not just shoot them down. Doctor is preferably one who does not perform the surgery, nor the typical consultation, and is as unbiased as possible about the situation. This might be difficult to find.)
4) I am not a doctor. Do not quote me. This is not a medical consultation. Just what I know of what can potentially happen. (I'm just a freshman pre-med kid, but I know what I know)

To FrozenTwins: I'm not even sure Tolkien could have created such a thing as the female psyche. George Lucas neither. We're freaks. Please love us. That's (seriously. I'm completely serious. I'll even say it in chatspeak. SRSLY SRSLY SRSLY) all we want from guys. And, well, everything that would ensue. But that's all. It's not much. Srsly! >v< And dude, if this guy can figure out and understand how the female mind works w/o any questions or doubts, cc me on that memo. I'd certainly love to understand my brain. Do guys understand everything about the male brain? xD I'd like that memo too, kthx.

Don't try to handle this yourself. Have him seek the counsel of a bishop or other church leader and kneel down and pray with him. Lead a humble prayer yourself and follow the spirit. He needs to realize that his actions have eternal consequences.