Let's assume that instead of leetcode, the company had decided that the qualifying test for programmers would be sketching a vase of flowers (about as relevant as leetcode for commercial programming).
Your friend would have spent three weeks practicing their sketching, right? And they would have made a decent drawing at the interview. And that would have the exact same ROI as spending those weeks on practicing leetcode.
The point is not that studying leetcode is good ROI. The point is that large orgs are testing for something that isn't relevant to commercial coding.
This is classic Streetlight Effect [0]. Just because it's easy to test leetcode, and it appears to have something vaguely to do with coding, it gets used.
Like I said in other posts, I would use the "pick a bug and fix it" method. But I'd also screen down from 100 candidates to ~20 or so before getting into technical interviews.
And, again, I don't think leetcode will scale well to 100 interviews either. You need technical staff in that interview room, and if you subject your team to 100 technical interviews using leetcode they're not going to be happy about it.
Your friend would have spent three weeks practicing their sketching, right? And they would have made a decent drawing at the interview. And that would have the exact same ROI as spending those weeks on practicing leetcode.
The point is not that studying leetcode is good ROI. The point is that large orgs are testing for something that isn't relevant to commercial coding.