Yes, for single trips only. As long as this is done before the end date of the policy and there are no claims made or pending on the policy. These do not have to be referred to TIF on a case by case basis.
However, you will need to establish where they are and why they need the extension, as if they are currently in a country on Foreign & Commonwealth Development Office (FCDO)'s travel ban list, please see this article for more info: https://holidayextras-insurance.zendesk.com/hc/en-gb/articles/360015869880-Is-a-customer-still-covered-on-their-policy-if-the-FCO-advice-against-travel-after-they-ve-left-the-UK-