parents having sex

HELP BROO I just walked in on my bestfriend's dad having sex with someone who is not her mother bc i went to get lotion in their bathroom buh the door was kinda opened ion know if I should tell her or nah bc she'll be heart broken or like not believe me OR idk I can jus keep it a secret without her knowing but which one will be a true friend... a white lie or the truth that will hurt her:(