In the newer systems 802.11g is dragged down but not to the level of 802.11b, the dragged down bandwidth is usually 2-3 times more then 802.11b
I do not think that just the presence of another 2.4GHz signal will necessarily drag the 802.11g down. It needs to be associated with your connection.
However, his strong signal might affect the general quality of your Wireless Network since both use 2.4GHz. If your Barricade is Wireless ask him to switch off his system for few minutes, and see what the difference with and without his signal is.