The recommendation to not drink water before the specified time is in order to avoid getting an indeterminate result perhaps due to inadequate oral mucosal transudate.
Since your result was negative, there was enough fluid for the rapid test to check for antibodies to HIV, you might not need to retest.
This is a really confusing question with a lot of missing information.
1) you said he took two tests. When did he take the other one?
2) how long was it from his last unprotected/condom break encounter to when he took the Oraquick?
He couldn't wait out the full 30 minutes after the sip of water to test?