All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
There is no doubt that homosexuality is one of the most obnoxious and despicable sins and it is one of the great major sins which necessitates sincere repentance to Allaah. It is confirmed that the Prophet said: "Indeed Allaah has cursed the person who does the act of the people of Loot." [Ahmad] Therefore, it is an obligation on your friend to hasten to repent to Allaah before he dies.
Your duties towards your friend are as follows:
1) Remind him of Allaah and instruct him to repent to Him; this is among doing one's duty of enjoining good and forbidding evil. Indeed, the Prophet said: "He who sees something abominable should change it with his hand (physically); and if he is unable to do so, then with his tongue (verbally), and if he is unable to do so, then he should (abhor it) from his heart, and this is the weakest level of faith." [Muslim] So you are obliged to forbid him from doing this sin if he is still committing it. Saying that you are supporting him, if you mean that you are helping him financially to do this sin or providing him with any help for this purpose, then you have participated with him in sin, so fear Allaah and repent to Him, and you are obliged to stop your support to him immediately. Allaah says (which means): {Help you one another in virtue, righteousness and piety but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment.}[Qur'an 5:2]. However, if you mean that you are helping him financially to spend on himself or to treat himself, then this is a good thing.
2) You have to conceal his sin, as the Prophet said: "He who conceals the sins of a Muslim, Allaah will conceal his sins in this life and in the Hereafter." [Al-Bukhari and Muslim] The Prophet further said: "Indeed your Lord is Hayyiy (which implies bashfulness) and Sitteer (which implies concealing the sins), and He loves concealing the sins and bashfulness." [Abu Daawood] So you have to conceal his sin and not inform his family about it, it might be that he will return to Allaah and repent from his sin. However, if one is sure that concealing his sin could lead him to transmit this disease to other people he socializes or lives with, or to his family when he returns back to them, then in this case you can inform them about his disease without informing them about its cause. Besides, you have to endeavour to call him to be steadfast on the religion of Allaah and threaten to inform his family if he does not stop committing this obnoxious sin, it might be that he would take heed. Moreover, his relatives or friends or any person who knows him should advise him to stop committing this sin. As regards your question whether or not he is a Muslim, the answer is that this sin is one of the great major sins, yet, it is not a sin that takes the person out of the fold of Islam, and the one who commits it does not become a non-Muslim, unless he believes that this sin is lawful.
Furthermore, we advise this person with this affliction to fear Allaah and be steadfast on His religion and stay away from dissolute people. Indeed life is very short and it is not eternal. Allaah says (which means): {And be afraid of the Day when you shall be brought back to Allaah. Then every person shall be paid what he earned, and they shall not be dealt with unjustly.}[Qur'an 2:281]. Moreover, he has to know that he will stand in front of Allaah naked and bare-footed in a Day equal to 50,000 years. So he has to take the initiative to repent, be righteous and save himself from Hellfire; we seek refuge of Allaah from it.
Allaah knows best.