It is necessary for her to learn English
"For her" in this sentence is not part of the adjective "necessary". It is part of the infinitive clause "for her to learn English". This is evident from the fact that this whole constituent can be moved:
[For her to learn English] is necessary
In English, an infinitive clause can have a subject, but it has to be preceded by "for" as a marker.
Two special features of the to-infinitival construction are, firstly, the for that introduces the clause if it contains a subject and, secondly, the to itself that marks the VP. (Huddleston & Pullum, 2002: 1181)
It is rude of her to ignore him
In this case, "of her" is a complement of "rude", which means that it is part of the adjective phrase "rude of her". Adjectives that take this kind of complement are generally those that describe personality, style, and attitude, eg:
Note also that adjectives that fall within this category typically occur in combination with it + extraposed subject: cf. "She is afraid of death", but not *"it was afraid of her to be dead". (The adjective "afraid" takes an of-phrase complement but does not belong to this category.)
In short, "for her" in the first sentence is a subject of an infinitival clause, and "of her" in the second sentence is a prepositional phrase functioning as a complement of "rude".