In order for this to work, the software would have to be running on your router, which is what feeds both of your systems.

There are Linux distributions designed around being packet shapers, but these would essentially take the place of your router, and kind of defeat the purpose.

Many firmware routers include the ability to do this, but no thought is given to the ability of the hardware to handle this task, so it's probably best not to even attempt it on a standard router. You can buy gaming oriented routers with this feature. They cost more because the hardware is more substantial to handle the extra processing load. That would be your best bet if you can't just get the other person to curb their actions.