Shops are not legally bound to give refunds just because the buyer has changed her mind - only if the goods are faulty, not as described or not fit for the purpose.
So Sara's suggestion might not work. However, if the shop offers refunds within a certain timescale for any reason, then there will be no problem buying the cheaper one from the website and returning it to the store.
If she no longer has the receipt and the t shirt has been reduced in store, she may only get back the reduced price, not the price she paid.