"But it is not possible for the firm to say "We sacked Bob because he sexually assaulted one of our staff"."
Why not? The event in question happened in a public setting, while her boss was acting in something of an official capacity (it was a work related event). To me, that demands a public acknowledgement and treatment of what happened.
They leave themselves open to law suits. He hasn't been questioned by police, nor arrested, nor tried, nor convicted.
Innocent until proven guilty is a firm principle.
Accusing someone of a sex crime will have very serious consequences for that person.
She has said that she doesn't want to involve legal processes. When the company publicly says that he assaulted her they force the legal processes to start - either he sues for defamation or police get involved.
They appear to have provided support to her.
She could go to the police and file a report. It's traumatic, and unlikely to have any effect.
But it is not possible for the firm to say "We sacked Bob because he sexually assaulted one of our staff".