Cheapest way is if they also have a computer, you can then use one of the Messenger services - no cost to either of you.
There are companies who provide for internet phone call - try a Google search using your topic heading. They are not free.
Alternatively, get yourself a Post Office Phone Card which allows really cheap calls from your landline to almost anywhere in the world, eg Canada and US for 4p per minute. Depending on where your friend goes he/she may be liable to pay for incoming call (on mobile).