or was it an 'off the shelf' model?
If it was the latter your friend simply has to tell Carrera that she no longer wishes to proceed with her order, and that she wants a full refund. As long as she does this within seven days she's entitled to the refund, and there's no question of a restocking charge - it's illegal to charge one. There's nothing to restock anyway.
If a computer is being built to a buyer's specification and the finished machine is what's called a 'non standard item' the distance selling regulations do not apply, and if your friend's machine falls into this category things are not so straightforward - she must negotiate with Carrera.
In the event of an order cancellation under the distance selling regulations the supplier MUST be notified of the cancellation in writing - a telephone call is not sufficient.