You'll probably have to tell her the truth tomorrow.
Literal
Tomorrow her [to-に] truth [possessive-の] thing [object-を] say-must [probably-でしょう].
~なければならない expresses obligation ('must'), and ~でしょう adds conjecture ('probably'). 本当のこと ('the true thing') is the standard conversational expression for 'the truth.' The combination of obligation + conjecture creates reluctant acknowledgment.