Warfarin apparently tastes like vanilla. So, I can see where it could be in food without raising the victim's suspicion. But, I would think that it would have to be in large quantities, and I'm not sure it could act that quickly.
I take Coumadin for a blood clotting condition and I find it very hard to believe that this is the culprit. When you first start on the regiment, you have to take shots in the stomach along with the pill form for over a week. The shot is the instant acting and the pill is the slow acting. Once the pill catches up and can keep the blood at the thinness required for your condition, then you can stop the shots. If they stood up and collapsed with bleeding, etc., it must be something more potent.