From the start of the novel, when we are described the character of Silas Marner, we realize that he is a man who is inherently "sane and honest", loyal, "fervent", and very true to his church.
Shortly after this description comes that of his best friend, whose connection to Silas is so deep that the townsmen would dub them "David and Jonathan". However, Eliot is careful to juxtapose the depiction of best friend William Dane in a way that evokes animalization, which is the opposite of personification.
When learning the description of Dane, the reader cannot help but imagine him as a cat lurking in the distance, watching for his prey. Eliot gives us everything, from the "slanted eyes", to the focused look in his face, to the sardonic and somewhat sarcastic undertone of Dane's behaviors toward his so-called best friend, Silas.
We first learn that he is a lot into himself, rough on those who are weaker, and seemingly someone who likes to attract attention. He thought himself smarter than the others, as well.
The real name of the friend was William Dane, and he, too, was regarded as a shining instance of youthful piety, though somewhat given to over-severity towards weaker brethren, and to be so dazzled by his own light as to hold himself wiser than his teachers..
Interestingly, it is Silas, and not William, who ends up earning the popularity and the "glamour" of popularity among his peers. Surely, this is not something William takes lightly.
To add to this, Eliot adds that, from the two, Silas may have been the more popular one, but not the strongest. Silas has a problem with doubting himself, and making himself insecure. Perhaps William was his beacon of strength, and this is what kept the men so bonded. Still, William's intentions were far from noble. Eliot prepares his next move with a rich characterization of both Silas and Dane that leaves little to the imagination when it comes to what a recipe for disaster their friendship can be:
The expression of trusting simplicity in Marner's face, heightened by that absence of special observation, that defenseless, deer-like gaze which belongs to large, prominent eyes, was strongly contrasted by the self-complacent suppression of inward triumph that lurked in the narrow, slanting eyes and compressed lips of William Dane.
This is everything the reader needs to know to understand how, after that, William plunges the proverbial knife on Silas's back, frames him for a robbery that he did not commit, disgraces him in the town of Lantern Yard, and takes Silas's fiancée too! Therefore, the answer to the question is that William Dane is absolutely NOT a good friend. He is a foe disguised as a friend. He is a very bad man.