Consumer legislation says that if a fault develops within six months of the date of purchase the law assumes that the fault was there (inherent) when the transaction occurred, unless the retailer can prove otherwise.
In those circumstances the retailer is liable, unless of course it can be demonstrated that the fault is the result of misuse.
PC World will almost certainly choose to repair the machine, and that must be done within a 'reasonable' period. The law doesn't define 'reasonable', but in this context 28 days would seem to be right.
If the laptop goes for repair your friend would be wise to back up any important data files and delete anything confidential from the hard drive.