If you are a GUARANTEED first rounder, then yes, you sit. But he is not and I cannot tell you how many times guys have gotten bad advice, only to end up lower than these mysterious agents whispered.
I don't have the expertise to be definitive but I suspect if he played a balls out game, and he breaks the record, it could improve his draft score just a bit - and that may be the difference if he is on the #33 or #34 bubble between second and first. Its certainly a risk but it may be worth it in his case to try to ensure first round. If he isnt on the bubble though, meh, sitting may be best.
Agreed 100% if he is being told he is for SURE a 1st round. Only reason to risk the injury IMO is if he is near the bubble - thats maybe a $2-3m difference IIRC. But I would not blame him in the least if he decides not to risk it.