。
She's a die-hard coffee lover.
Literal
She [topic-は] root-from [genitive-の] coffee-faction [is-です].
コーヒー党 uses the suffix 党 ('party, faction, club') in its idiomatic sense for a 'devotee, partisan, member of the X camp.' Originally referring to political parties (政党), the suffix has become productive for taste preferences and lifestyle identifications: コーヒー党 (coffee lover), 紅茶党 (tea lover), 甘党 (sweet-tooth), 辛党 (one who prefers savory/spicy or alcohol). 根っからの sets up the 'thorough, bone-deep' framing — she's not a casual fan, she's in the coffee camp by inclination.