By the sounds of it there is something in the BIOS or thereabouts that is picking up the HDD SMART info and giving the warning. You could go into the BIOS Setup and see if there is an option to disable either the warning or SMART altogether (there is often something in there).
However, just because the system seems to work OK doesn't mean that the disk is not starting to fail. Modern HDDs incorporate technology to circumvent failures and keep working - but only up to various limits. Once one of the limits has been reached it will, effectively, become a hard failure. Ultimately, it is your (or your brother's) call as to whether you want to run the risk.