Tilly and Watts have been dating for four years and plan to get married next year.
According to the Daily Mail, one of Powell's family members said she was "absolutely devastated, but she has known the risks that were always involved".
"She grew up with a father who was a stuntman and has followed the same career. She knew that they always faced risks on any dangerous stunt."
Watts has previously worked on Hollywood blockbusters like Star Wars: The Last Jedi, Spider-Man Far From Home, Mission: Impossible - Fallout, Jurassic World and The Hitman's Bodyguard.
All production on Fast & Furious 9 was halted immediately after the accident and it's unknown when filming will resume.
Fast and Furious 9 is due for release next year.