Sorry, you'll have to find some other way to eavesdrop on your "colleague." :-)
The AP, being a layer 2 device, forwards traffic based on MAC address. IF you change your IP address, you won't change your MAC address. Now, if and when your PC generates an ARP reply, the upstream router will update its ARP table. But now your colleague will no longer get data, so she'll know that something is up. Her computer may also generate an ARP reply, in which case she will get the traffic instead of you. Your two computers will "fight" for the data, and likely neither one of you will receive enough useful data.
This assumes you maintain your association with the AP as you change your address. Depending on the client, that might not happen.