If the computer is working properly, and is 'as described' in any shop poster or specification card displayed the retailer is under no obligation to accept a return.
Some shops operate a goodwill policy where unwanted gifts are concerned, especially around Christmas time, but they do that purely to get people to buy in the first place. Even then, goods must be unused, and in the original packaging.
The seven day return rule applies to distance selling only.
Marg7's comment about YouTube buffering is valid - it's caused by bandwidth issues, not by the computer.
The machine your mother bought is generally a solid performer, and has been well reviewed, although its small form factor is going to limit future expansion options.