Have a look at
http://www.truphone.com
I have google talk installed on my pc and the truphone app installed on my nokia mobile.
works very well. when l am at home, people can call me on the truphone app that has an 07978 xxx xxx number, or on google talk if l have my PC on. if your friend is the states he would get a US number when he registered with truphone.
When l am away from home with no wireless access the 07978 xxx xxx automatically diverts to my normal mobile number. However if l am in a wi-fi hotspot it can connect as if at home.
truphone to truphone/google talk/sip calls are free. As well as 0800 numbers.