Don't spread grammatical urban legends. "Who" and "whom" are both perfectly correct here, and I dare you to find a reliable source that says "who" isn't allowed.
Reader's Digest, your grade-school teacher, and anything written before lingustics was a science do not count as reliable sources for this purpose.
I'm not much of a stickler for grammar, and I don't particularly care. I'd be happy if "whom" died off.
However, it's still taught in school, in college, and in any modern grammar book. It's clearly coming up on the cusp of disappearing, but it certainly hasn't been written out of the books just yet, and I'm not sure why you're pretending that it has been. This sort of mistake would have been corrected in basically every English class I ever took in the last five years. It would have been corrected not just by the professor, but also by a majority of peers if peer grading was utilized.
My rule of thumb is to basically never use "whom" in speech and to conditionally use it in writing, depending on the context and how formal it is. I'll generally use it correctly when writing posts like this on the internet as well, but anything less formal and I won't care. I feel like the threshold of minimum formality to justify using it is always rising, and perhaps in the next decade we'll see that threshold rise up above the level of formality of, say, a paper for a literature class. But that definitely hasn't happened yet.
This reflects my general belief that grammar is a spectrum. It's less about the mindless application of rules and more about pattern recognition. Rules of grammar are best understood as being part of a context, and that context is not just the rest of the sentence or paragraph; it includes the social context as well, the mode of communication (written, verbal, etc.), and basically everything else you could think of. My description above is an example of how one rule -- that rule pertaining to the usage of "whom" -- morphs according to context.
To summarize, grammar is the art of satisfying two constraints simultaneously: (1) making sure nobody else thinks you've made a mistake; (2) making sure your efforts at (1) aren't going to distract people.
(1) could be seen as maximizing acceptance of your grammatical decisions, and (2) could be seen as minimizing social offense. "whom" is a great example, because in certain contexts people might recognize it as correct but they'll think you're only using it because you're trying to show off how smart you are. You've unnecessarily distracted them by using it, which violates (2).
I'm not pretending anything. I'm not saying you shouldn't use "whom". You have a reasonable view of different registers, except for the part where you believe that "whom" has to "die off" before "who" can be fully correct in this usage, or that "who" is any kind of mistake.
The words are just both correct. Some people like to argue from authority that there's some rule against using "who" there, except -- as is the case for many prescriptive bogeymen -- the supposed authority actually doesn't exist at all.
Ask anyone who scientifically studies the actual English language for a living -- fenomas's comment links to a good source at Cambridge, which is very well reputed in linguistics -- and they will not cast a single bit of doubt on using "who" to introduce a relative clause. Everyone uses it, including very good writers.
It's easy to caricature linguists as always saying "as long as you can be understood, anything goes". And that's unfortunate -- nobody here would put up with misunderstanding and making fun of the work biologists do, for example, but for some reason everyone thinks that being literate makes them qualified to disagree with linguists.
There are linguists who make recommendations about not just how to be understood but how to write well, based on lots of study and observation on how people use language in all registers, and if you're looking for an authority you should be looking to them. If you do look -- I'll point at fenomas's comment again, because he's the only one who actually linked a source -- you'll find they don't support your position.
It's too bad you took a lot of English classes that wasted time on correcting this word. (Editing out a harsh and hasty conclusion I came to about your English professors here.) If I assume the best of them, I'll say it was a pedagogical exercise. Sometimes it's important to distinguish different cases. This isn't a situation where you have to distinguish cases in English, but you can.
So maybe your classes told you to use "whom" there kind of like a class in object-oriented programming might tell you to create a "class Rabbit" that inherits from "class Animal" -- as a way of showing that you understand the concept, not as something that you'd actually have to do in the real world.
My apologies, I learned something about relative clauses today. I still like my general theory that I posted but I accept that I was simply wrong about this particular case.