To be clear, the traveler did not use a credit card. She apparently used a debit card and that is the crux of the problem as holds, deposits or whatever term is used on a debit card mean that the funds are actually withdrawn from the account.

Presuming that there was no damage to the room or any other unpaid amount, the friend should simply contact her bank (presuming that this is a UK account) and have the bank become involved. If the funds are owed, the bank will deal with this in short order.

Do not waste time with the property, Marriott or anyone on the merchant vendor side.

For the benefit of others, do not ever use anything other than a credit card when paying for services to be rendered in the future. This especially includes room deposits and other forms of "hold." If this had been a credit card, it would have been a simple chargeback and a slight nuisance.

Also for the benefit of others, the days of paying anything appreciable for calls are long past. Use Skype or any of the other dozens of VOIP services available. At most, these calls cost a few cents and are largely toll-free.