According to both USATF & World Athletic rules, pacing by someone not registered in the race is not allowed.
You actually have two separate but related points-
1. If the race is sanctioned by USATF or WA, they have an obligation to DQ a runner who violates the rules in order to maintain fair play by all athletes - especially those they would be in contention for an award. Unfortunately this may not be the case in some races that don't even know there are rules of the sport!
2. If an athlete is disqualified by a race, their performance will not be listed in the official results which are submitted to the Boston Marathon.
fyi - Race courses are certified not the race. Here in the USA, events are sanctioned and the course is certified. Other parts of the world may use other terminology for "sanction" but "certified" always refers to the accuracy of the race course.