Depends on their rules about employing people with previous convictions or particular convictions.
Has he ever been asked to disclose anything previously of his own free will?
It may make things difficult if he's been asked before, be it on an application form or other paperwork etc... and has not disclosed due to the dishonesty.
Similarly if there are any rules in place that mean he should have disclosed it of his own free will.
Could he approach his bosses before the check is done or the results come back and be honest and explain if it's definite it will come up on the CRB check?
What were the circumstances of the assault?