In most instances an app would have to be installed on the device before-hand in order to track it. lookout mobile security is a good free one, but once again it would need to be installed first before you can track it.
also: "There are cell-phone GPS tracking services for commercial users, and cell phone location is built into the national enhanced 911 service. However, secret cell phone tracking by individuals is not legal and is not available through the carriers. Be careful if you are seeking to implement secret cell phone tracking, because there are a number of scams centered on this practice."
there is one thing i might suggest however, if he is the account owner, or authorized user on the same account, your friend could always try looking into the call logs for the device and pin-point through the records an approximate location.
also... if its his kid and is missing, why not involve the authorities? They can certainly locate the child...