The real killer, without a shadow of a doubt, was Helen.
While he was talking to Olivia, she broke a mercury thermometer and poured the fluid into Ashad's drink. Since mercury is more dense than water and they were drinking using a straw, Helen lifted her straw to get only the part without mercury, while Ashad unfortunatedly only drank the part that contained the substance.
Thankfully, it wasn't a great amount of mercury, so Ashad recovered in about a month.

(No Miis were hurt during this story)