By Zakariyya Adaramola

The one who made a mistake or sinned should be advised or corrected sincerely for the sake of Allah, without debasing him or her.
So debasing or openly condemning someone for a sin he committed is detested in Islam, according to the scholars.
It is reported in At-Tirmidhee and other collections in marfoo’ form [i.e. that the Prophet said]: “Whosoever condemns his brother for a sin (he committed) will not die until he has committed it (i.e. the same sin) himself.”
But Ibn Rajab said this hadeeth is referring to a sin, of which the person who committed it has repented from.
One of the righteous predecessors of Islam, Al-Fudhail (rahimahullaah) said: “The believer conceals (the sin of his brother) and advises (him), while the evildoer disgraces and condemns (him).” 
This is what Al-Fudail has mentioned as being from the signs of advising and condemning - and it is that advising is linked to secrecy while condemning is linked to publicizing.
It used to be said: “Whosoever commends his brother (towards doing good deeds) at the head of a gathering, then, he has condemned him.”
The Salaf (the righteous predecessors) used to hate that commending good and forbidding evil be done in this manner. Instead, they loved that it be done privately between the one commending and the one being commended, for indeed, this is from the signs of sincere advice. This is since it is not the goal of the one who is advising to spread and publicise the faults of the person he is advising, rather, his goal is only to put an end to the evil that he has fallen into.
 As for spreading and exposing someone’s faults, then that is from the things that Allah and His Messenger, sallAllaahu ‘alayhi wa sallam, have forbidden. Allah, may He be Exalted, says: “Verily, those who love that the evil and indecent actions of those who believe should be propagated (and spread), they will have a painful torment in this world and in the Hereafter.
And Allah knows and you know not. And had it not been for the grace of Allah and His mercy on you, (Allah would have hastened the punishment on you) and that Allah is full of kindness, Most Merciful.” Surat-un-Noor: 19-20.
The ahadeeth concerning the virtue of keeping the faults of others secret are many.
Some of the scholars would say to those who were commanding towards good: “Strive hard to conceal the faults of the sinners for, indeed, exposing their faults shows a weakness in Islam.
It is for this reason that spreading someone’s evil and indecent actions is linked to condemning. And they are both from the affairs of the evildoer, since it is not the goal of the evildoer to put an end to the fault nor that the believer avoids that fault or defect. Rather his only goal is to spread and publicize the defects found in his believing brother, and to destroy his honor. So he initiates that and repeats it. And his intention is to belittle his believing brother by exposing his defects and bad qualities to the people so that some harm can fall upon him in this world.
But as for the person that is sincerely advising, his aim in doing that (advising) is to eradicate the faults found in his believing brother and to help him avoid it.

Culled from Daily trust http://www.dailytrust.com.ng/news/islamic-forum/how-to-advise-a-sinner/113319.html