My friend started to use a road
Assuming that you don't mean "friend" in the same sense as "my friend has a rash", my first question would be what value you are adding to the process by acting as an intermediary, rather than your friend posting himself first hand.
I read on google that if you have more than 9 NIPs, you should seek the advice of a motor offence solicitor. I wonder if it is best he do that straightaway?
I'm going to go ahead and say that you never read that anywhere. You probably read that if you have 9 or more points and are one conviction away from a totting ban, that you should seek the advice of a motoring offence solicitor. 9 points is a significant number. 9 NIPs simply means that you are royally screwed. From a clean licence, 5 NIPs (when you were the driver) would result in a minimum of 12 points (unless you had a viable defence), if you were offered and took an SAC for one of them. So, presumably your attention to detail and ability to accurately convey what you read somewhere is the value you are adding to the process...
Legal professionals will variously tell you that you don't need a lawyer or do need a lawyer, based largely on whether or not they are charging for their time.
What is the best course of action? Complete the forms and return them as requested to confirm that he is the driver but should he include a cover letter to explain why he missed the speed limit signs, or will that make it worse?
Speeding is a strict liability offence - if you exceed the speed limit, you commit the offence, regardless of whether or not you intended to break the law. On the face of it, a covering letter explaining that you didn't intentionally broke the law, but you committed the offences due to committing a separate offence of driving below the standard expected of a careful and competent driver, is not the most cunning plan ever devised.
He has 27 days from date of service of each NIP (28 days beginning with...) to name the driver. N.B. The response must be served (delivered) within that timescale, not merely posted within the 27 days. Absent the most contrived Royal mail delays, he should probably have received all the NIPs he's going to receive well before he needs to send the first response. How many NIPs he actually receives would affect aspects of the best courses of action. Unless
The FPSU/CTO is mostly a sausage machine, and there is no formal process to appeal or otherwise attempt to persuade them not to proceed - BUT, in the past there have been cases where a human has exercised discretion in response to a covering letter regarding the potential consequences of what was essentially a single mistake being repeated, and due to the offences all being committed before the first NIP was received, being unaware of the error and the need to raise the standard of driving. Not many, but if you don't ask, you don't get. We have also had posters asking us if it was worthwhile sending a covering letter - which translates to "I know I'm facing a 6 month totting up ban, but I really can't be bothered in trying to do anything for myself to try to avoid the ban, unless there's a good chance of it working". In my own experience, if you phone the FPSU/CTO and speak to them nicely, once they get over the shock, they can be quite helpful and human.
Regardless, it is generally advantageous to send all the responses together in the same envelope if you can.
For multiple alleged offences in the same 20 limit, I would also suggest obtaining a copy of the Traffic Regulation Order (TRO) from the relevant authority, and checking that it is valid (or posting it up here and asking us to see if we can pick any holes in it). The chances of the Order being defective are small, but if the Order is defective, there never was a 20 limit.
As damage limitation, *if* he ends up getting say 10 NIPs (or 9 NIPs which apparently is a significant number) and all pleas for clemency fall on deaf ears, he would want all the charges to be heard at the same time (other than possibly accepting 3 COFPs to keep the fines down). If he is going to get more than 12 points, he wants to get the additional points at the same time - reaching 12 or more points triggers a totting up ban (subject to Exceptional Hardship). If he tots up to 12 or 30 points at that hearing, it is the same process. He either gets a totting ban and those points are wiped, or his exceptional hardship argument succeeds. If this happens when he has more offences still hanging over him, the points go on his licence, and/or he can't use the same exceptional hardship argument a second time within 3 years.