Can we really hack a "facebook" account? If yes then please do tell me HOW?

[Angus :)]

You really shouldn't be doing this, and you can just ask facebook to review the situation through facebook support. Problem solved.

But facebook support can take ages, and by then, the damage will probably be done.
Hypothetically speaking, if you were to gain full control his main email (I'm talking reset all his security backups like security question, mobile phone numbers and recovery emails), by guessing his security question, it IS possible to get his facebook by taking the newly acquired email address, putting into the login section, clicking "forgot password" and following the steps to reset the account via "his" email.
Ooooor, once you have his email, you could guess the security question on his facebook (often the same one), and reset the other emails. Perhaps, it's possible to use the "Identify photos of friends" option facebook has in place to recover hijacked accounts.
Of course I would STRONGLY recommend you don't do that, and I trust you would NEVER do such a thing.