It's like floppy drives, they think only a minority of users use them, but they are wrong. There is still a large amount of people still on dial-up and still people who use a floppy drive.
If you friend mentioned that they were still on dial-up, you would of thought they would have checked or told them that they don't come with internal modems now. I am surprised they didn't try to sell him/her an external usb dial-up modem, do pcworld stock them?
I suppose if you friend is not pleased with the machine, they could return kit as it is " not fit for it's purpose" i.e. connecting to a dial-up service. They might fit an internal modem for your friend for free:)
They still seem to put them into laptops, I take it this is a desktop. You could always get a cheap one and bung it in or take the one from the old machine and put it in the new machine.