I spent years cooking brisket "done" but not tender (not leaving on long enough for fat to render), and I could never figure out how come it always seemed tough. (Pre-internet days.) Finally talked to Mom who always cooked great and tender brisket in the oven. She explained cook at 225 for many hours until tender. I suspect your friend did what I used to do.
Regards, Riz58 Char Broil Akorn, Bandera, and Masterbuilt Electric Smoker