This will depend on what Trevor is actually saying to people when telling them that Craig committed the crime.
Scenario A. If Trevor is saying that "Craig was convicted," or "Craig is guilty" (in a way that reasonable people would understand implies a criminal conviction), then Trevor is at a real risk of losing a defamation suit, as the falsity of the statement will be easily made out by Craig (or in Canada, where falsity of defamatory statements is presumed and the burden shifts to the defendant, Trevor will be unable to prove the truth of the statement).
Scenario B. However, if instead Trevor:
- says merely that Craig did X, Y, or Z (e.g. "Craig lied to the bank about his property to get a big loan"; "Craig pushed me"; "Craig stole my mail"), or
- says that Trevor saw Craig do X, Y, or Z (e.g. "I saw Craig push A"); or
- uses the language of a criminal offence without implying a conviction or a strictly technical connotation (e.g. "Craig assaulted me"; "Craig spread hate speech"),
then it is totally open to Trevor to try to show that those things happened on a civil standard in order to defend himself in a defamation action. The existence of a criminal acquittal on a charge relating to those facts will not be a barrier to arguing the facts are true on a civil standard.
the failure of a prosecutor to satisfy a standard of proof beyond any reasonable doubt has no relevance to proceedings in which a plaintiff must only prove a case on the balance of probabilities.
- McLean v. Pettigrew,  S.C.R. 62. The Supreme Court of Canada found that it was open for a plaintiff to argue in a civil suit that the defendant's driving had been "wrongful" according to the laws of Ontario, even though the defendant had been acquitted in a criminal case.
- Re Stinson and Medical Council (1911), 18 C.C.C. 396 (Ont. H. Ct. J. (Div. Ct.)). There is no estoppel in civil proceedings relating to matters that were previously the subject of criminal proceedings that resulted in acquittal. "If a libel action be brought against the publisher of a newspaper for calling a man a perjurer, the defence of justification is not met by simply proving that the plaintiff has been acquitted." That statement was made by Riddell J., at first instance, and affirmed by the divisional court with reasons written by Meredith C.J., Chief Justice of Ontario.
A slight correction to terminology. I notice you suggest that "that he committed the crime is now proved to be true on the civil standard," but that is not what would be happening in Scenario B: the court in the civil case would not be concerned with proving that Craig committed a "crime."