How do you know if you have a scratched cornea and what should be done?

that sounds like a scratch and yes it hurts a lot. I've had this with hard contacts its one of the things they are more prone to than soft. I'd leave it a few hours and it will get better. If it is getting worse tomorrow then it may have an infection - hopefully your mum washed her eye out with saline not water. This is painful , but eyes do heal quite fast. Give it 24 hours.