As there was no 'distance selling involved, there's no 'cooling off' period.
That's confirmed by the policy statements of firms like SpecSavers:
https://www.specsavers.co.uk/help-and-faqs/how-do-i-cancel-an-order
The only legal argument that
might possibly weigh in your mother's favour is if the glasses could be viewed as 'not as described'. (i.e. if, having been told that only the cheapest style was acceptable, the salesperson made it appear that your mother was being offered such an option, even though it wasn't the case). I think that it might be hard to prove such a case though, so your only real chance of getting the order cancelled or amended is to rely upon the goodwill of the shop's manager.