Matthew 18:16 But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established.
I would like to share some ideas about how to deal with sin and failure in the lives of our church members and especially leadership that might at least give you something to think about. This is assuming that the person that you are dealing with has acknowledged their sin and desires to get right with God. I do think that I will start with what I would do along the way to even get them to that point.
1. First do not believe what people tell you about another preacher, leader, or Christian. You will soon find out that it is easy to accuse another brother and we were instructed to not receive an accusation against a brother without witnesses
1 Timothy 5:19 Against an elder receive not an accusation, but before two or three witnesses.
That not only applies to elders, pastors, or leaders but to all people:
Deuternomy 17:6 At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death.
Deuternomy 19:15 One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established. Remember that there can be false witnesses also.
Deuternomy 19:18 And the judges shall make diligent inquisition: and, behold, if the witness be a false witness, and hath testified falsely against his brother;
Deuternomy 19:19 Then shall ye do unto him, as he had thought to have done unto his brother: so shalt thou put the evil away from among you.
Step one is to be sure that you have really investigated to find out if the brother is guilty. I suggest that rather than accusing or seeming that you want to harm your brother that you go to him and that you ask him what he has to say about the accusations that you have heard. The conversation would be as kind and as pleasant as it could be because our goal is to always restore our brother.
Ask him like this; I have heard some things that concern me and I wanted to ask you to help me understand and know the truth about this things.
If he denies it then you probe further and further until you have an idea about the truth. If he does not confess and you do have witnesses then you will have to bring them out but it would be much better if the Holy Spirit would convict him and cause him to recognize his sin and to get it right as soon as possible.