Did the volunteers do right or wrong by abandoning charitable work and telling their colleagues to stop volunteering in an association where one of its officials was discovered to have committed theft? And is it sufficient for them to merely inform the owners of the association, who decided to only dismiss the thief and not expose him?

The stolen money must be recovered from the thief, and he should be reminded of Allah. If he refuses to return the stolen money, his case should be referred to the Sharia (definition) court. As for informing the volunteers whose money was stolen, this is necessary because their responsibility has not been discharged, and the betrayal of an agent does not nullify the obligation of the owner. However, there is no need to inform the general public about the theft, as this might discourage people from donating.