She chose to be offended. There was nothing inherently offensive about the comment as it's reported. It was apparently a private comment to a friend.
If I'm eavesdropping on some friends talking amongst themselves then I'd expect to hear all sorts of crass lewdness TBH. If I then choose to be offended perhaps the lookout is on me, that I should stop eavesdropping other's conversations.
The situation at hand — to borrow your metaphor — is like someone coming out of a haunted house, seeing someone across the street dressed as an axe-murderer (but clearly in fancy-dress), then crossing the street to harangue them because one should know axe-murderers frighten them and that some how the happenstance of your co-locality gives them the right to control over your attire.
If you don't like the content of private conversations that you can overhear, as an adult, in a public setting, then your choices are to put up with it, move out of earshot, or ask the people to censor themselves.