I'd say it's Justin 90% and Ayew 10%.
Justin doesn't get himself into a tricky position or puts Ayew in a difficult position either if he takes responsibility and gets rid.
His hesitation means not only does he not know what to do, but other players have no idea either. Villa pounce on him like a fly on shit, and it pays off because he's then panicked into making the only pass available when in fact there were better options. This is why Justin should shoulder most of the responsibility for that goal. Ayew less so.