The Arthur she is referring to is Arthur Weasley, who, in Harry Potter and the Order of the Phoenix, was attacked and almost killed by Nagini, an attack which Harry Potter saw in his mind, and alerted people at Hogwarts.
Today marks the 18th Anniversary for the Battle of Hogwarts in J.K. Rowling’s final book Harry Potter and The Deathly Hallows.
Remus Lupin was actually briefly brought back to life when Harry Potter used the Resurrection Stone, along with Harry Potter’s parents and Sirius Black.
The author revealed on her Twitter page early this morning that she first decided to kill Remis Lupin while writing Harry Potter and the Order of the Phoenix.