What permission do I need to obtain from the user in order to access a post from one of the users friends to another of the users friends wall?
Here is a screenshot example. user_1 & user_2 are both friends of the user. user_1 has posted something on user_2‘s wall. From my application I need to get this content from the authorization of their mutual friend.

At the moment, the permissions I have from the user include this:

With these permissions I cannot access the content. The following error occurs
GraphMethodException: Unsupported get request.
How can I access the content?
That error does not relate to permission but your object ID.
What URL are you trying to load?
It mean the graph cannot find the object ID you are loading.
But you might just want these permissions:
read_stream, friends_status
and read user_2’s /feed