Your friend has two choices: either leave it as it is or try to find out what’s going on.
If he leaves it as it is and the police do not consider a satisfactory response to the s172 has been received, your friend will be prosecuted for failing to provide the driver’s details. He may have a defence to that charge – it depends what sort of response was made and whether he can show that it was submitted. If that does happen, whether he is convicted or not, the speeding charge is a dead duck as the police have no evidence to prove who was driving.
If he makes some enquiries he might find out what is causing the apparent confusion, get it put right and the driver will face action for the speeding offence. This could result in a course, a fixed penalty (£100 and 3 points) or prosecution in court (depending on the speed/limit and the driver’s eligibility for the out of court disposals).
The advantage of doing nothing is that, if the police do not have satisfactory evidence of who was driving the speeding charge is a nullity. The disadvantage is that if the RK is prosecuted and convicted for failing to provide the driver’s details he faces a hefty fine, surcharge and costs (probably around £1,000) and six points. He will also have an endorsement code (MS90) which will see his insurance premiums rocket for up to five years.
The advantage of sorting it out is that the RK will be off the hook once the police are satisfied that the driver’s details have been provided. The disadvantage is that attention then turns to the driver who will face action for speeding.
If I was the RK I would want to find out what the problem was rather than playing hardball by insisting he has complied with the request.
If you tell us the alleged speed and limit, together with the RK's driving record, we can tell you what the likely outcome of the speeding allegation will be. But nobody on here will encourage your friend to commit a criminal offence.