To start with, I wouldn't go to a party with 100-150 kids who are drinking and doing drugs. If I somehow did, I wouldn't let somebody walk up to me and push me to the ground...there is no such thing as coming out of nowhere. He came from somewhere and your friend didn't notice it.
Assuming I already made the first two mistakes and was on the ground, I would probably sit there, and if he advanced on me, I would probably try to break his knee. A person can exert a lot of force with a kick when they are laying on their back.