Prosecutors allege MacNeill drugged and drowned his wife, Michele MacNeill, 50, on April 11, 2007, so he could pursue a relationship with Willis.
Prosecutors presented phone records on Friday that they said indicated the couple exchanged text messages during Michele MacNeill's funeral, which they both attended.
"This was a very informal, discreet thing," Willis said of her relationship with MacNeill. "I think he was trying to keep it quiet."
Willis was the latest in the prosecution's arsenal of star witnesses to testify at the trial. Last week, MacNeill's oldest daughters, Alexis Somers and Rachel MacNeill, testified.
MacNeill's defense lawyers said heart problems were a contributing factor in the death of the former beauty queen and mother of eight, and that the once-prominent doctor isn't a murderer.
The medical examiner has never determined a cause of death for Michele MacNeill.