Pleading guilty to the speeding would be a rash move, there is then nothing to stop them continuing the prosecution for the S172 offence and leaving him having to defend it (possibly unsuccessfully).
It is better to plead NOT guilty to both charges and in the mitigation offer to state he will plead guilty to the speeding if the S172 is discontinued, sometimes the single justice will action that otherwise it may require a (short and simple) court visit.
This is a well trodden path, simple to execute and in all but one case (where the defendant made an ass of himself and frankly got what they deserved) successful.
