Bill had helped with the set up for the Christmas dinner, despite getting the impression from his wife that he shouldn't. But still, he couldn't make himself sit by and do nothing. Brooding had a habit of being self-perpetuating and Bill didn't want to get sucked into that cycle.
And he very consciously avoided the thought that keeping himself busy was just another unhealthy way to deal with things and avoid his grief. He'd done it after Fred, he'd done it after his attack, he saw no reason to change methods now.
By the time Ron arrived, he'd been chased out of the kitchen already and resorted to doing some paperwork he'd brought home from work. When he heard the knock at the door he set everything aside and came out of the bedroom to see Fleur greet Ron. "Hey, you made it." He headed for the living room as well so they could sit together.