WHAT?! While it's not specifically mentioned that Scabbers AKA Peter Pettigrew who is a person, was with Ron, he's carried around and mentioned enough in the books that we can safely assume he was present. Trelawney was therefore the 14th so when Dumbledore stood up to get another chair... Uh oh.
The whole: “When 13 people dine together, the first to rise will die,” Trelawney predicition comes up a few times in the books and it always makes us sad. Like when Sirius was first to stand during the dinner of 13 in Harry Potter and the Order of the Phoenix. And then Remus stood up first following Moody's death in the Deathly Hallows, where 13 people were sitting in the Weasley's living room.
Oh JK Rowling, you've done it again!