You would copy Foul Play.
From the Compendium:
Q. If my opponent used Zoroark's "Foul Play" attack their previous turn, could I play Guzma to bring up a benched Pokemon and have Sudowoodo's "Watch and Learn" attack use "Foul Play" to copy one of the new Defending Pokemon's attcks?
A, Yep, that works. (Mar 1, 2018 TPCi Rules Team)