Ballistic descent from the point of failed a failed booster separation has happened before - in 1975:
There's another write-up here: America Space:'Creeping and Unpleasant': The Near-Space Experience of Soyuz 18A
That hit more than 21G. The astro/cosmonauts train in a centrifuge up to 8G. I've no idea what G-force this one hit, but hopefully survivable without permanent injury preventing them from taking part in later missions.