Can an iPhone 4S from AT&T be used on another phone network?

I wanted to give my friend my old iPhone 4S, but I have AT&T and she has Verizon, so I was wondering if it's possible if they can switch the phone over to Verizon or is that not possible?

In this case it is possible, though its up to Verizon. Should enquire on if they would.

Most company's will allow you to pay to supply you with an unlock code for the phone. Once unlocked the phone can be used on any of the networks that it supports. Call ATT customer support and ask. Cost normally around $35 dollars, that is still a good deal for you friend.