If their phones are not tri-band phones (many arent) then they wont even be able to pick up a signal in the US.
The USA uses a different mobile phone system to here in the UK and some phones do not have the capability to work in the US.
I also think any mobile calls will be prohibitively expensive (particularly on a pay as you go), so are not really practical.
In the US you can buy "phone cards" very cheaply that allow your to phone all over the world. They are available in newsagents, Wal Mart and so on.
I bought one when I was in Florida and we found we could talk for AGES using one of these phone cards. You can use them from any hotel phone or normal phone in a villa or whatever.
Basically you dial a US number, then dial the UK number and it routes you through to the UK. It think they work by using "unused" phone networks that the big companies (Bell etc) are not using at that time.