I can just picture Danny getting woozy and Howard saying: Here, take some of your mom's tea. 15 minutes Danny was worse, 1 hour he was dead. (7:30-8:00).
Howard goes to the john, washes ONE of Anna's cup and tosses it in the bathroom basket or in the LADIES Public Restroom basket on the floor.
Three cups, three people left. What's the problem??
Not only that, if the throwaway cup was found, it would have Danny's and Anna's DNA and prints if any could be found. And Howard's of course because HE went to Wendy's.