In which case DNS flushing should solve it. Anyway kinda hard to say without knowing what router he is using. Alternatively you can also try and manually releasing and renewing the IP on the router. My guess though in all honesty is that there is just some temporary ISP problem.
Also the connection doesn't have to mean he has legitimate access. You can try this yourself by turning on MAC filter on your router. This will allow you to connect to the network with full signal strength, yet returns only garbage to client requests not on the mac list.