If you are right to play at weddings, then drinking enough to make you lose inhibitions is simply stupid because you won't sing well. Any singer who has to drink to sing lets it impact on their performance. Fine, if your style is certain kinds, but a killer for others. You need to develop a technique to deal with it. For many, the feeling comes from intimidation. Seeing the people watching and perhaps talking about you, or you interpreting what you see and thinking they are thinking about you. You need to take charge. A girl friend of mine is a solo singer, and does the posher solo singer stuff with a band, where everyone looks at her, not the band. She suffered badly, until she read that a famous singer had the same trouble, and while she sang, imagined all the audience sitting there naked. The big fat guy, the wrinkly skinny lady. It's sort of a game, and it makes you smile, and she tried this and it worked. Sometimes stage fright high tens your awareness and makes you perform better. You just need to get it under control.
In this case, you are doing it for a friend. Keep that centred. It's for your friend, not these people. Some people can overcome stage fright, some can't, ever! I've done shows with big names who have thrown up in a bucket in the wings, but once they start, it's fine. For your friend, have fun, and enjoy it if you can. Everyone is on your side. The more comfy you feel, the better. Please, do it sober. Booze rarely improves your performance, and if you drink enough to lose your natural fear, then you are pissed! If it's important, do it sober.