Setting up a computer to provide internet access to another computer via Internet Connection Sharing will work off multiple IP addresses.
Hmm... Ok. Internet Connection -> First Computer. That computer gets an IP address from the Internet Connection device and sees the Internet Connection as a Gateway. Now... if you set up ICS (Internet Connection Sharing) to share that internet connection across a second network card (Wireless or wired, doesn't matter) ICS will want to work on a different internal IP range, and set up that first computer as ANOTHER gateway.
I'll put it another way. Let's say her laptop is connected via ethernet cable to the internet connection in her apartment. Her laptop gets an IP address of 10.0.0.200. You set up her laptop to share that internet connection across her wireless card. This will require also enabling ICS. ICS will assign the IP address of 192.168.0.1 to the Wireless Card, and make it a Gateway. If you connect your laptop to this new Ad-Hoc network, you will get an IP address on that new range (192.168.0.2 maybe) and your computer will now be seeing 192.168.0.1 as your internet gateway.
Some details? The computer that is going to be in the middle has to have two network cards (two wireless, one wireless one wired, two wired) where one is connected to the Internet Source, and the other connects to the other computer.
The computer in the middle will not act as an Access Point, just passing out IP addresses in the existing range that is offered by the internet source. It will indeed act like a router, starting a new IP range.