From your main router, connect the ethernet cable from it to one of your macbooks and run a ping test. Your Internet Service Provider should have a dedicated ping test on its website. Run it first on your macbook, then connect your girlfriend's macbook to it and run another ping test, making note of the download speeds and upload speeds to see if there is a marked difference between the two macbooks. There shouldn't be, but it's worth doing a test anyway.

Next, phone your ISP and request Remote Access. One of their technicians will take over and test the sort of things they know while talking to you on your phone. Make sure your router is not anywhere near metal objects and electrical sockets/plugs.