Soontobe - Like RuddyNorah, I am a bit confused by this. If police question you about an incident, you should know what is alleged to have happened.
In terms of the HR investigation, police investigations take so long that it is common for the HR investigation to continue at the same time. The HR investigation is not looking to anything like the high standard of 'beyond reasonable doubt' that the police would need to prove if they brought criminal charges. They are looking (roughly speaking) to the civil standard, which is to decide, on balance, whose version of events is most likely. Likewise, something could be sufficiently serious to merit disciplinary action at work, but not warrant criminal action (e.g making a suggestive comment) That means that, even if nothing goes ahead with the police, disciplinary action could at work. As a result, work don't necessarily need to wait for the outcome of the police investigation. They can do their own. It's just that, if there was a criminal conviction, that would be strong evidence if work then did a later disciplinary.
However, everything Flowery has said is correct (as usual ). If your DH is being questioned, I would expect him to know the allegations and have enough detail to respond - i.e. what is alleged to have happened, when and where (the last two might be approximate - e.g. a couple of weeks ago in the office). If HR try to question him without this basic information then, as Flowery says, he should explain that he cannot answer.
However, I have to admit that something isn't adding up for me. As Ruddy says, the police will explain the allegations to your DH when they question him. Likewise, a decent HR department will explain the allegations. I am not trying to say that your DH has something to hide, but are you sure he isn't trying to protect you from the nasty details of what has been alleged?