Negredo netted his second from the spot before Manu del Moral completed the rout to set up a last-four clash with either Atletico Madrid or fierce rival Real Betis.
By the 13th minute the game was already shaping up for a Munster rout as Foley broke the Viadana line on the right wing and passed outside to Anthony Horgan.
Hun Sen could rout them at leisure, though in the run-up to July's national elections, he could find it useful to have a few bogeymen still in reserve.