Praise be to Allaah.
What is prescribed in sharee’ah is for you to advise him and point him in the right direction, and urge him to fulfil the trust. If he does not respond, then you have to refer the matter to the appropriate people, because Allaah says (interpretation of the meaning);
“Help you one another in Al Birr and At Taqwa (virtue, righteousness and piety)” [al-Maa’idah 5:2]
And the Prophet (peace and blessings of Allaah be upon him) said:
“Religion is sincerity (naseehah)” three times. It was asked, “To whom, O Messenger of Allaah?” He said, “To Allaah, to His Book, to His Messenger, to the leaders of the Muslims and to their common folk.” (Narrated by Muslim in his Saheeh).
And Allaah is the Source of strength.