hmm .. which is the main computer connecting to the router?
Let's give an example. Normally, the router will have an address such as:
192.198.1.1
Next computer first on network might have an address such as:
192.198.1.2
And next computer an address such as:
192.198.1.3, etc.
If she has more than one account on her computer then it may count as a different number for each account. (Or if she has peripheral devices connected such as iphone or ipod etc.)

And yes, her computer MUST have a different address to yours. Each computer on the network must have a different address with the router as the lowest denominator ip address.
If you had 5 computers connected to the router, then the fifth computer might have an address such as 192.198.1.6

If you were both connecting using same address, then yes that would cause a connection problem. On an apple mac I think the network ip address is automatically allocated. I suspect same is true for Windows. For Linux, I have no idea. I did read in one of the above links, you do actually need to manually edit the connection files. Perhaps .. an alternative might be to set her computer up to connect to the router first? With Windows XP might make it easier. But you may still need to manually edit a connection file .. I don't know. I am totally Linux dumb. lol

Networking around a router can be confusing, also depending on the router software, and I guess also depending on the computer systems involved, since Linux does not have such ready made assistance software for it that comes with the router software support etc.

From my understanding, as long as all the network connections each have a separate connection ip address that comes after the router's address, they may even start from 192.198.1.50 for the first computer in the network for example. Usually the router will have a default internal ip address such as 192.198.1.1. This can be changed, but not recommended (unless clashing with a secondary router of sorts.)