The only person "at fault" here is his friend for telling him what interview question he received and I'd expect companies know this happens, which is one of the benefits of having further interview stages in the first place.
* Communicated with a colleague (communications/interpersonal networking) * Thought hard about it (applying previous knowledge and experience) * Looked up and found relevant information (knowing where to look) * Combined the above to come to a solution. (and had the skill to put it together)
By coincidence the practice practice question was also the one that was asked at the interview. If the interviewers had asked a different question, similar steps would have needed to be applied[1].
The question itself was an obvious practice question. There are many decent answers to the problem. What they were looking for were the knowledge, skills and experience that needed to be applied in the process of solving the problem. The point was to see if the candidate had those abilities.
I'm fine with using a practice question for this. A real world problem often involves a lot of real world noise, which makes it impossible to efficiently resolve within the constraints of a 60 minute chat.
I am -however- fully open to an argument that perhaps some form of Goodhart's Law ("When a measure becomes a target, it ceases to be a good measure") may have struck again.
[1] Perhaps not the exact same set of skills, given the constraints of the interview setting. One COULD argue that the interviewers accidentally picked up on more than they were looking for.