She'll wait day after day, but he probably won't come back.
Literal
She [topic-は] coming-day [also-も] coming-day [also-も] continue-waiting [probably-だろう] [but-が], he [topic-は] return will-not [probably-だろう].
来る日も来る日も ('day after day') is a literary set phrase expressing monotonous repetition. だろうが connects two conjectures contrastively: 'she'll probably wait, but he probably won't return.' The sentence has a melancholic, literary quality.