I wouldn't really call them "friends" if they want you to prove yourself, but here are some tips
1)try to give him a hard chop in the neck, it should take him out
2)if he try's to start the fight by pushing you, throw an elbow at him
3)aim to him him around his side's, it will take his air away
4)Put a protector on, in case he try's to hit you there(disregard this if you are a girl)
5)aim generally for his sides, nose, and neck, cover yourself, and be calm, wait for you're chance to strike, if you know some judo the grab him and slam him against the floor, finish him with a good kick to the side
Good Luck :wink: