one of my friend all his moves as poison type and i battle him with a steel type to win the battle but why isnt it working , why does his pokemon use "doublekick"

In this game, pokemon can learn any move irrespective or whether they are able to learn it in actual game or not.
To beat his steel type, try using V-create, as it will give super effective damage and its power is more than any other move super effective on steel-type.

u got it all wrong , he has a golett with all the moves that cannot effect a steel type pokemon , but when i try to battle him , his golett uses moves which are not there in his moves

i.e.

His golett's moves are



but when i battle his golett

[you must login to view images]

(As You see , his Golurk used Double Kick though it is not included in his moves)