It actually never happened in a game. In Resident Evil 5, when you are riding the boat with Sheva, you will see a cutscene where Jill (BSAA) and Chris (S.T.A.R.S) are in the Spencer Estate to arrest Ozwell E. Spencer, one of the founder of the Umbrella Corporation but find Wesker there and Spencer is dead. They try to arrest Wesker instead but Wesker now has superhuman strength and nearly kills Chris and Jill. When Chris is about to get killed, Jill tackles Wesker and the fly out the window over a large cliff (+3000 feet) and Jill is presumed dead after 3 months of active searching. You can actually read where I got this info in the Library section of RE5 and click on Files and click on Jill Valentine.