It happens so often in trials. BB knows that she can't stand up to cross examination so, bad as it looks ( and it will go down badly with the Jury ) it would be even worse for her to be on the stand, getting tangled in knots by the Prosecution. Lesser of two evils ( sorry not the best quote there ! ) for her to remain silent.
As for Malik, we just don't know really. So little has been reported. I think the Prosecution have done enough to at least show he must have been aware of what was happening to Sara...but whether they are able to prove his active involvement, I just don't know.