igadget: Collision domains aren't an issue in a switched environment. Each connection between switch port and NIC, or switch port to switch port, is a collision domain. Collisions are only an issue now if someone throws and old hub in the mix somewhere.
As far as latency is concerned though, that's very true. For a business, this could be a serious bottleneck if the switches are low-end. For most home networks, you could do a lot worse and it would never be an issue.
Getting back on topic, I would agree with all comments about finding a new installer. The installer should be able to explain to the client how they are doing the job, and why that's right for this situation. It should never be left up to the client to guess what they need. This sounds too much like an electrician looking for the old "The specs should have said what type of cable to use and how to terminate it" excuse when things don't work properly in the end.
