Can you use an AT&T iPhone on Verizon network?

I want to buy an iPhone form my friend but she got it from AT&T and I have Verizon can I still buy it from her and use it on my Verizon line?

No because AT&T and Verizon are two different networks.

Absolutely not. AT&T phones are GSM only while Verizon is CDMA, they're incompatible.

If you unlock it, yes.