The chances of successfully defending a speeding charge on this basis are virtually zero.
12am and 12pm are both equally incorrect in describing 12noon or 12 midnight. However, it would have to be shown that the prosecution is unable to say at what time of day that the offence took place. In fact, all that is being disputed is the annotation used on the summons to describe the time of day. This issue will be quickly covered by the prosecutor in his outline of the case against your friend. Any evidence then produced to prove the case will also easily establish whether it was noon or midnight that the offence was alleged to have taken place.
The suggestion that an alibi could be provided to show your friend was elsewhere is therefore a non-starter as the time of the offence will have been established by the prosecution evidence. The summons itself does not form part of that evidence and, as has been said, its details can, if necessary, be amended and this can be done up until the prosecution closes its case.
Unless your friend is prepared to employ somebody like celebrity lawyer Nick Freeman (�Mr Loophole") to give the case a run for a not inconsiderable sum of his money, he would be well advised to plead guilty and gain the maximum discount available for an early guilty plea.