Switching to contactless at King’s Cross

My mum is taking a trip down to London and staying with a friend in Enfield. She already has a ticket to King’s Cross. It looks like the easiest and cheapest way to get to Enfield Chase is to use contactless changing at Finsbury Park. Am I correct that to use that she would exit through the barriers from her long distance train and then come back in to use contactless? Or is there an obvious validator at the platform?

There aren't any validators on the platform - she will need to exit the barriers and re-enter. Most services to Finsbury Park use the higher-numbered platforms (9 to 11) that are behind a separate barrier line in any event.

It may be quicker to use the Victoria line to Highbury & Islington and change there for a service to Enfield - it's a cross-platform interchange at H&I. Admittedly it's a fair way from the National Rail platforms down to the Victoria line at Kings Cross, especially if you follow the signs rather than taking the shortcut.

Alternatively she could take a Thameslink service from St Pancras across the road - these are more frequent than the services from Kings Cross which are (generally) only half-hourly.

If she has a Senior Railcard, she could get it added to an Oyster card at any Tube station for a 1/3 discount on the fare (if off-peak).

Thank you for the advice.
Apparently the friend has now offered to pick her up from Oakwood station, so that's a simple trip on the Piccadilly line rather than something involving changes.