The owner of the WiFi network sets the type of WiFi security used. The most usual nowadays for home routers is "WPA2 Personal". But there are older types, like "WPA" and even "WEP". The older types have security vulnerabilities, if I remember correctly. This setting is typically in the settings for his home wireless router. (So he might not even know what it's set to.)
It could be that he has the WiFi security set to one of the older (flawed) types, and the modern Apple products refuse to connect to it (because of the security issues). Perhaps your Windows box is older, or doesn't care, and connects anyway. But this is just a guess.
Since your Windows machine is connecting, there is likely somewhere in Windows that you can examine the WiFi security setting (but I don't know where -- maybe WiFI connection properties?). (On a Mac, you can see connected networks' security setting in SystemPrefs>Network, select WiFi, then click Advanced.. Then in WiFi tab, see the Network Name and Security columns -- but this won't help you until the Mac successfully connects.)
Anyway, if Windows says it's connecting with WPA2, then my guess is wrong. If it says WPA or WEP, I might be right. (?)