KISS Listener Needs Love Triangle Advice

"Karen" is a student that had a random fling with her professor. She says that the fling was inspired by curiosity and thrill but that she knew she would never really take him seriously. Karen explains that she cut things off with the professor when she’d found out that he was married and had a son. Months later she begins to date a fellow student named "Matthew" and immediately finds out that her new boo and the professor are acquainted when she sees them say hello in passing. Karen and Matthew have shared a few months together and things are starting to get a little more serious as he has invited her to be his guest at a wedding where he will introduce her to his friends as his girlfriend. Karen’s conscious is bothering her for not telling Matthew that she is more than familiar with his friend, the professor. Most recently, Karen joined Matthew at a school function where the professor saw the two hand in hand and sent text messages to Karen confessing his love for her and his feeling of betrayal because she had moved on to one of his friends. Karen remains silent and doesn’t respond to any communication initiated by the professor insisting that when they parted ways she was clear that her intentions weren't to be a mistress to a married man. Karen believes that "boys will be boys" and she’s worried that in a moment of vengeance & jealousy the professor will tell Matthew to cause trouble in paradise.

The question is: Should Karen tell Matthew about the fling with the professor….or just completely not worry about it?