Well i don't think that he would have any problem at all because he got an I-20 with Valid OPT Period. Its just a matter of wrong date placed in his OPT card. If he is working as a contractor(Through consultancies), he even don't have to worry about it. However if he is employed directly, just show them the I-20 if they asked it for work autorization. IF somebody mention about OPT card just tell them he is waiting for it..

But be sure to get another OPT card with correct dates...