What does Joe teach Pip and how do Pip's sentiments toward Joe at various points in the novel serve as a barometer of Pip's values?"Great...

Most importantly, Joe in Dickens's "Great Expectations"  teaches Pip the meaning of real love.  From the time that Pip is small, Joe loves him, protecting him from "Tickler," and the wrath of his wife, Mrs. Joe.  Joe takes Pip with him and encourages the boy in his schoolwork. He instills moral values into Pip.  After Pip has told lies about his visit to Satis House, Joe tells Pip,

'There's one thing you may be sure of Pip,,,namely, that lies is lies.  However they come, they didn't ought to come, and they come from the father of lies, and work round to the same.  Don't you tell me no more of 'em,Pip.  That ain't the way to get out of being common, old chap.'

Yet, after scolding Pip, Joe encourages him, too, tell Pip he is "uncommon," not "common" as Estella has insultingly told him. Pip is "an uncommon scholar."

Joe accommodates Pip's wish to have Miss Havisham as his benefactor and dresses up to appear at her mansion.  Obviously, he is very uncomfortable, but he acquiesces to whatever is asked of him because he loves the boy. When Pip becomes a gentleman and Joe's appearance before Herbert embarrasses Pip, Joe tells Pip that he will not come anymore. 

You and me is not two figures to be together in London...I'm wrong out of the forge, the kitche , or off th' meshes.

But, he asks Pip to visit him at home.  Nevertheless, Pip grows to feel himself too high to visit Joe.  Still, the man does not chastise Pip, and when they next meet, Joe is as loving as ever.  In fact, Joe is the greatest friend that Pip has; he is the one who comes to Pip's aid after Pip is burned from rescuing Miss Havisham.  "Ever the best of friends; ain't us, Pip?" Joe comforts him.  Unselfishly, Joe returns to the forge as he is "Not wishful to intrude."

After Joe has come to nurse him, Pip is ashamed of himself.  He is also

'ashamed to tell him exactly how I was placed (financially).  He would want to help me out of his little savings, I knew...and I knew that I must not suffer him to do it.'

Clearly, from Joe Pip has also acquired integrity as well as the meaning of unselfish love.  With this sense of integrity, then, Pip visits Biddy and seeks to make things right with Biddy, asking her to marry him.  However, he learns that Biddy is to be married to Joe.  Unselfishly, Pip tells Biddy that she has "the best husband in the whole world..." and he says the reciprocal to Joe.

Armed with the valuale lessons learned from Joe, Pip assesses his other relationships, as well.  He reflects,

We owed so much to Herbert's ever cheerful industry and readiness that I often wondered how I had conceived that old idea of his inaptitude, until I was one day enlightened by the reflection that perhaps the inaptitued had neer been in him at all, but had been in me.

Out of friendship and love for Herbert--as taught to him by Joe--Pip goes to Miss Havisham and procures a solution to Herbert's financial woes without his friend's knowing, thus saving Herbert his gentlemanly pride.  From Joe, Pip acquires the good character to honor his friends.