It's not the subject of the sentence, but "who" is a pronoun in nominative case because it is the subject of the appositive phrase. You wouldn't say "him also recovered", but "he also recovered" in any case. You use "whom" where "him" would make more sense and "who" where "he" would make more sense.
It's very pedantic, because it's the subject of a pronounal phrase which is itself an object in the sentence. Personally, I'm not a fan of "whom" in general. It's not technically necessary, can always be replaced with "who" in modern English, and I see it used incorrectly almost as often as correctly.
This sort of thing is easier for German speakers, who have to inflect all pronouns correctly for every part of speech. English speakers largely get who wrong because there are very few words in the language that make you have to keep track of more than primary subject and object. It's not their fault, really, it's just an archaic construct of English from when the language was more modal.
It's very pedantic, because it's the subject of a pronounal phrase which is itself an object in the sentence. Personally, I'm not a fan of "whom" in general. It's not technically necessary, can always be replaced with "who" in modern English, and I see it used incorrectly almost as often as correctly.
This sort of thing is easier for German speakers, who have to inflect all pronouns correctly for every part of speech. English speakers largely get who wrong because there are very few words in the language that make you have to keep track of more than primary subject and object. It's not their fault, really, it's just an archaic construct of English from when the language was more modal.