[Garfie]

I got the impression they knew before the eviction show, and that housemates had drawn their own conclusions during the day.
Knowing Roxanne and her need for attention, my guess is she got too cocky and carried away, and revealed the 'assault' to someone like Nick or Hardeep- or maybe they heard about it as from other housemates- and they looked at her story logically, and pointed out it was full of holes.

For instance, if she had been punched repeatedly in the ribs by Ryan, logic would suggest she would have been taken out of the house for medical checks, and Ryan would have been immediately removed. My bet is that Nick or Hardeep pointed out the logic, cast doubt on her story and questioned her version of events, thereby making other housemates doubt her story too.

Or maybe it came out when Ben confronted Ryan?

Either way, I definitely got the impression from the highlight show that the housemates had already figured it out, and had made their feelings known.