Lol!! They'll try anything once, no such thing as long as he kept within the speed limit, but they may have a case if they can prove he was being reckless at the time i.e Reckless driving due to unneccesary accelertation. which is difficult without another offence like speeding, hence reckless driving is such a killer when it comes to re-insurance
Yeah as above there is no such thing as a safe rate of acceleration. However, they probably pulled him for shooting off with a little too much haste, which can be dangerous, and they can pull you over if they feel it is indeed dangerous. I suspect they told him they pulled him for accelerating too fast, but didnt suggest that he would get charged with the offence called "Accelerating too fast"