Hi Victoria. Its a real challenge to identify apples as there are hundreds..possibly thousands of varieties. But there are some good guides on the web, and if you google ‘uk apple varieties’ you might get some ideas as to what it might be. It would help to know what the fruit looks like when fully ripe, inside and out, its texture ( mealy, crisp, soft, fragrant, sweet, acidic etc) and whether it has any colour in the flesh.

Thankyou! I am asking for a friend so not sure of the ins and outs I’m afraid- I was thinking it resembles a cox that we used to pick in our dads garden?

You didn't put your location on your profile - but some areas have an "apple day" in the autumn where you can take specimens for identification. If you have an allotment association, gardening club or similar in your area they may know if there is likely to be one within reach.

Or check this website for further info
https://www.awarenessdays.com/awareness-days-calendar/apple-day-2018/