A man in dark armor and a pale pink cloak spotted with blood stepped up to Robb. "Jaime Lannister sends his regards." He thrust his longsword through her sonâ€™s heart, and twisted.
Actually I think it is merely an ironic reference of Roose's part to Jaime's parting words to Roose Bolton before leaving Harrenhal for King's Landing. Jaime of course knows nothing of the Red Wedding to come or his Father's hand and means his words in an entirely different context.
Both parties left Harrenhal the same morning, beneath a cold grey sky that promised rain. Ser Aenys Frey had marched three days before, striking northeast for the kingsroad. Bolton
meant to follow him. "The Trident is in flood," he told Jaime. "Even at the ruby ford, the crossing will be difficult. You will give my warm regards to your father?"
"So long as you give mine to Robb Stark."
"That I shall."
And so a terrible and ill-fitting fate laid down for Jaime, ever the one of misattributed deeds, by a misunderstanding and perhaps the one and only moment we see humour from Roose Bolton.