Which person is talking (his/her) way out of it, the one who hears the news or the one who explained having herpes? (I have a friend who is recently going through this. This friend has huge anger and still likes the guy, while angry. No, I am not "the friend".)
Given it's the second, is there an expectation whatever changed explanation would be believed? Is the herpes the truth?
given it's the first, there are many avenues, including sudden diarrhea/food poisoning - needing to check email - needing to stop at the store before it closes, bla bla.
There is the possibility of having a genuine talk about this, but that doesn't seem to be your story direction.
Is this a romantic comedy?