She or her in a relative clause where she is both subject and possessor
It should be her, because it is the object of the preposition of.
Her is not part of the relative clause: the relative clause only includes who and what follows. The role of who in the relative clause (subject) has no bearing on anything outside the relative clause.
Antecedent (her) and relative pronoun (who) must normally agree in number, but their syntactic roles are independent of each other and may be different, as here.