Smeagle vs Mankey
Mankey uses Low Kick.
Smeargle uses Sketch, it learns low kick.
Mankey uses Low Kick again and the battle is over.
Lets say it was a wifi battle, would Smeargle still know Sketch or Low Kick?
The sketched move returns to sketch after the battle. Source: http://pokemondb.net/pokebase/5102/sketch-in-wi-fi-battle
It depends, if this is a Wifi battle, it'll only last for the battle.
If non-wfifi though, it will last permanetely unless Smeargle re-learns Sketch and replaces it with the learned move.