Add 001 and drop the first 0 of their Number. Your friend needs to dial his/her international code (which I think is 011) then 44 for the UK and drop the first 0 of your number. I hope that makes sense. :-)
Each country has a code ie UK is 44, France is 33, North America is 1, Australia is 61 and so on. From the UK we also have to dial 00 before the contry code and then the telephone number leaving of the first 0 on the number. A lot of countries dial 00 like us but some have something different, as I say I think the US and Canada is 011. I hope that helps.
Just a point, but do you have to drip the first zero of the US number? I was in the states last week and bought a cellphone to be able to phone some friends. The number I was allocated was based on the Orlando area (407) followed by seven figures, none of which was a zero. When I have called them from the UK, I hve dialled 001 but then dialled the number as given.
One quick point - if you are dialling an international number from a mobile, dial the access code as ** which will appear on the screen as +. This works like 00.
To clear some things up the + symbol can be used instead of the exit code (idd prefix) 00 and 011 for UK and US respectively. When calling the US you need to drop a 1 off the number not a 0 (you also need to add a 1 for between state calls so just leave the 1 there)