@contrex,
I agree with "whom" as a general ruling in this case.
However, it should also be said that a lot of twaddle is often spouted about "English Usage" because appropriate usage depends on context. What is acceptable in local
spoken dynamic usage (in which even the concept of "a sentence" is nebulous) can differ markedly from what is acceptable in general
written static usage, including the formal rules offered as a pedagogical "fix" for non-native speakers.
Much of the so-called "debate" about usage in this forum, including appeals to authority, is generated by ignorance of such contextual considerations.