If Brown had said that Wynne was a thief or fraudster, that would constitute libel. To say that she is "sitting in trial" is factual. To say that she is "on trial" (which are not his actual words) is unlikely to be judged libelous, since these words are often used in a figurative sense. 

But it is worth stepping back and recalling that Wynne did sit down in a trial to testify for her closest aides, who the Crown alleges did attempt to bribe a potential candidate, and that the aides' words are on tape, along with the words that they were offering the alleged bribes "on behalf of the premier." That should make even people who are used to such disgraceful conduct by the Liberals sit up and take notice. It's also worth asking the question why Wynne herself is not accused of bribery, since she was allegedly the perpetrator, and her aides merely the messengers, if their words are to be believed. If bribery is proven, they never would have had the authority to provide it, only Wynne.