I am not sure how she has it set up but my daughter uses my iTunes account here in New Zealand as well as her own account in Australia and has apps and music from both accounts on her iPod touch and her iPhone. I know she has to log in and out of either account when she updates apps but other than that everything seems to work alright.
I have also read of other people who have set up accounts in other countries to get apps that have yet to be released or are not available in their own app store and used them both on one device.