Quick GW question!!

So I started playing GW and bought nightfall chapter first..

when I finish the game I wanted to start farming to earn some cash but builds I have looked out I dont have atleast 1-3 of them and all needed...

so my question is I will buy prophecies to get the skills I need and after I finish the game will I be able to let my friend use the prophecies key while I am playing nightfall???

No
your friend would have to buy another copy

Each access key creates 1 account. You can't share keys or divide up the content or anything like that.

1 one question

Will I be able to travel back to prophecies maps when playing Nightfall character...

thank you

yah you just need to click on the boat and click on lions arch or kamadon

Each access key can only be used once. You can either use it to create a new account or add it to your existing account. If you intend to have access to Prophecies skills for your NF characters, you have to add the key to your existing account.

And in order to travel to other campaigns, you have to first do a quest. After that, you can freely travel back and forth via the map and the big boat icon located near the main cities.