Well, they can certainly joing you on your flight, providing they're on the same flight as you already

Otherwise, it'll be a call to Virgin - but to prepare you for the likely outcome, UK & US based Z fares are officially non-refundable and non-changeable.
The only way to "convert" it into a changeable fare will be to upgrade it to a FULL J fare. That'll be in the order of 2-4x the cost of the original Z [:0]