I am not sure if you are asking if I would encourage Hamlet or if I have a friend that reminds me of Hamlet. I think that he had a difficult decision to make and it tore at him. If he were my friend, I would want to help him out. However, I would not encourage him to make one move or another. It is ultimately Hamlet who needs to make the decision, not me. I would instead act as a sounding board, and listen to him and help him feel less alone, as he decided what to do.
Again as others have pointed out, we must put ourselves into the Elizabethan mind in answering this question. If someone were Hamlet's friend, he would be like Horatio since this is the type of person that Hamlet likes. Therefore, the friend would counsel him to be wary of the ghost; however, he would encourage him to listen to what it has to say, and to be observant and watchful in order to ascertain the truth of what it says.
I think that I would not want Hamlet to try to get revenge for his father's death. I hope that I would realize that doing so would ruin him. So I would try to convince him that the ghost was not real. I would try to get him to forget about the whole thing so that he could just have a happy life. This might not work, because the ghost might start to come straight to Hamlet, but I think that is what I would want for him. Hamlet is not the right person to be trying to get revenge so I would try to take that burden from him.
If you are asking if Hamlet were my friend would I encourage him to fulfill the requests of the ghost, then I would say, yes I would. I would encourage Hamlet to exact justice for his father's murder, but only after determining whether the ghost was a "true" ghost, or merely the devil in the disguise of a loved one who was trying to damn his soul.
Hamlet is clearly distraught over his father's death and even more so after hearing what the ghost had to say. Hamlet is disgusted by his mother's overly quick marriage to Claudius. He feels vulnerable and betrayed by his other friends, Rosencrantz and Guildenstern. I would behave as Horatio does. I would caution Hamlet in regards to the dangers of the ghost and others around him. I would help with anything Hamlet requested on me, such as helping to determine is Claudius reveals any guilt or says anything that could be interpreted as a confession of any kind. I would try to make sure that Hamlet knew he had a confidant whom he could trust.
Hamlet will never be settled until all these conflicts are settled for him. I would do what I could to ensure that Hamlet finds the resolution he needs to live a happy life. Like Horatio, I would be devastated if all of my efforts at being a good friend ended with my best friends death.
In order to answer this question, one must consider the era in which Shakespeare's play, Hamlet, takes place.
If a friend was living in Renaissance England, when the play was written, he would believe in the supernatural. Shakespeare's audiences believed in witches, demons, fairies, changelings, etc. It was generally accepted that the powers of darkness sent their servants to trick the souls of unsuspecting humans to their eternal damnation. And so the ghost might be good or evil. Elizabethans believed that ghosts could not make people do something, but that they could ask a living being to complete a task for them. If your friend cared about losing his soul, he might be cautious, as is Hamlet.
Another thing to consider is that killing a king in that era was considered a mortal sin. The Elizabethans believed that God chose who would be a king or queen: what was ordained by God could NOT be altered by human hands. If this happened, there would be a disruption to the balance of the universe. This is seen in Shakespeare's play, Macbeth, when an earthquake and an eclipse occur, and gentle animals become predatory—after King Duncan is murdered.
In Hamlet, we might consider that the appearance of the Ghost is also a result of this same disruption of the universe...if only Hamlet can prove that Claudius is guilty. Without proof, Hamlet might be killing an innocent man—simply based on the testimony of a ghost who could be a good ghost, but might be Satan's henchman. This is why Hamlet presents the play that reenacts the murder of his father, to see if Claudius will display guilt, thus giving Hamlet the proof he needs to act on the Ghost's request of vengeance for his murder.
In an Elizabethan sense, were your friend in the same situation, I believe he would hesitate to exact revenge on Hamlet's uncle, Claudius, until he had proof that it was a just killing.