I have 3 wired and and one wireless on my router with no trouble. When the kids get home from college and bring friends, I've had up to 10 computers running on the same router with out troubles.
It sounds to me like a setting in her laptop. I'd go through every setting in the networking setups and compare them to a different laptop in the house. You might find it that way, other than that, I'm out of my element.
Update, just talked with IT type son... he suggests looking and making sure the network is set to "obtain IP address automatically" under the network properties. If that is already set like that, he suggests that you remove/delete the network connection in the laptop, then re conect it having find a new address on its own. He thinks the one it has may be in conflict with your computer.