[Closed] network sim unlock codes......

got an HTC M9 from a mate as a 'try before you buy' to see if i can get the network sorted. when i put my sim in, it asks for an 8 digit network unlock code i think it was....

im on a three sim (rolling monthly contract) and the M9 was used on vodafone. my friend has got a new vodafone contract and swapped her sim over into the new phone.

what are my options for getting an unlock code? does she have to ring voda, explain the situation and ask for an unlock code so she can sell the phone on? is that free if she goes down that route?
do i search online for codes? or do i take it into a local high st telecomms place for them to do it? and how much is it likely to cost me?

thanks