Brendon, who played nice-guy demon hunter Xander on "Buffy" for seven seasons, has a recurring role on "Practice."
The Feb. 10 episode of "Practice" will also feature guest stars JoBeth Williams and Ann Cusack, ABC said.
"Charlotte's attacker, Lee Henry, arrives at St. Ambrose Hospital, injured by his girlfriend and in need of urgent care, and Sam and Charlotte must decide whether or not to save his life," the network said in a synopsis of the episode Tuesday. "Meanwhile, Susan's health takes a turn for the worse, and Addison's mother, Bizzy, begs her daughter to do everything in her power to help."
"Private Practice" stars Kate Walsh as Addison, KaDee Strickland as Charlotte and Taye Diggs as Sam.
Guest star Williams plays Bizzy, Brendon plays Lee and Cusack plays Susan.