I think Ms. Pierson issued a statement denying any affair with Senator Cruz. If she was telling the truth, the story must be a lie. If she was lying, then it follows that the story must be true. Since she is one of Mr. Trump's official spokespersons, her veracity is very important to the campaign. Therefore I would expect that if she is continued as an active spokesperson, Mr. Trump must be confident of Senator Cruz's innocence. Conversely, if he believes that Senator Cruz is guilty of the allegations, he would suspend or remove her as she would have no creditability to speak for his campaign. It is a dilemma and it will be interesting to see how it is resolved.
That’s just one explanation. We know that Trump has been aware of this scandal for some time and the fact that she has been implicated 2-3 weeks ago and still works on the campaign means he isn’t concerned. My guess is that she knows there is no concrete proof so it is “he said, she said” and that whatever she may have done with Cruz will not be a focal point going forward. Her status has nothing to do with Cruz’s guilt.
There's little doubt that this turn of events will benefit him greatly - so much so that one has to speculate, what did he know and when doid he know it? What Trump knew and when he knew it will remain a matter of speculation for a long time I suspect.