Sometimes cable modems only want to see one "mac" address on a modem. It is kind of involved to change it, but changing the mac address on the laptop to match the one on your main computer could fix it. That's what I had to do to get my laptop working on my mom's cable modem. You could also try enabling "internet sharing" on your main computer, but you would need some kind of hub. This is going to sound like gobbledygook, but maybe your friend would know how to try those things.