[laguerre1]

I have a functioning password for an email account, but I'm getting the "unusual activity" block, that requires a code from another email account that I don't have access to.

Does anyone know what criteria Yahoo's systems are using to determine "unusual activity"? Is it just IP, or IP + OS?

[FryPan]

It is most likely the IP. You need to either reverse proxy whoever you're trying to access or try and find a proxy that is around where he lives.

Also, he's probably got an email by now, so expect a password change soon unless you access it quick.

[prilux]

Yes, change password. The yahoo database was/is available online.