。
She's incredibly generous.
Literal
She [topic-は] very generosity [subject-が] is-good.
気前がいい is another 'body-part / inner-faculty' idiom: 気前 ('generosity, openhandedness') + がいい — 'is generous.' 気前 itself is a noun built from 気 ('spirit, disposition') + 前 ('in front of'); the modern reading is settled as 'generosity / liberality with money or hospitality.' Like other ~がいい/が悪い predicates, the trait is grammaticalized as 'X-quality is good/bad' rather than as a direct adjective applied to the person.