As long as the hand luggage is not looking way too big they don't really mind. The weight is more of an issue with them (6 kilos for pax in economy and premium economy).
Economy seats on Virgin are not great, they are certainly not well padded.
Virgin are not actually all that great either regarding water and drinks - you only get one with your meal. Pax need to go to the galley to get extra water.
Melsy cannot take any water or liquids more than 100ml through security but you can buy drinks on the other side. However, my son was told at the departure gate to finish off his drink (water) before boarding the aircraft.
If a pax is going Premium Economy or Upper Class they should hand over a Fasttrack voucher which will enable usage of the Fasttrack security line (this tends to be a lot shorter than the other line).
Food on the plane is not much to write home about (apart from the nice GU choc mousse though!) - would suggest she eats at the airport before boarding.
We did not get choc ice with the movies!!. Think we got a tub of vanilla ice cream instead.
The main things she needs to do on the plane is to relax and ensure that her visa waiver and customs form (she will in total have two forms to complete) are fully completed and correct before arriving into US immigration.
Oh and if she goes to Macys with photo ID (UK passport or driving licence) she can receive a visitors shopping card that will give her a further 11% off her shopping there!!.