[It's not a joking matter at all, and that takes away from the apology. Besides, it's more about the action of having done it - not what she's hidden from him. Which he'll type out next, taking a bit of time with it before he hits send.]
This isn't about what you did or did not tell me. It's about your actions.
[It probably comes across accusing or even hard in text, but ... he isn't, he's more tired than anything else. All the same, he's not going to switch. He'd rather not.]
[text]
This isn't about what you did or did not tell me. It's about your actions.
[It probably comes across accusing or even hard in text, but ... he isn't, he's more tired than anything else. All the same, he's not going to switch. He'd rather not.]