Even a Traxxas radio wouldnt do the trick. An adjustable radio would, but as everyone said, i doubt he has one.
I would suggest to him to sell it to someone with a Traxxas Rustler, or keep it as a spare, and get a "race worthy" speedo.
Otherwise, put a stick & tape between the radio and the trigger.
Either that, or give a penatly for driving in reverse, maybe a lap?. I know that this won't make him happy, but he won't do it again!