Q: How can I gain access, with permission, to friend's screen on her mac to help her?

All the material you need is here (and in the attached articles):

OS X El Capitan: Screen sharing overview

Most of the material above deals with computers on the same network.  You would have to explore the "back to my mac" path (which I have personally never used), I suspect, unless you ever bring the two Macs under the same network.

The easiest way is through iMessage.  Click the "details" button on an iMessage thread with your friend and you'll see a screen sharing icon.  Click that and have your friend give you permission to share/interact.