Testing a candidate's ability to express themselves is a good idea, but there are probably simpler ways of doing it than phoning a friend. If you wish to test communication skills, ask them an open-ended question like "how does a browser/OS/compiler/database work?".

I wouldn't like put one of my friends in such a position when answering an interview question. Nor would I like to be (partly) responsible for a friend's performance in theirs.

Anyway, surely a candidate that can do the question on their own is better than one that can't. And surely the friend that can answer the question is better than the candidate?

A technical interview question shouldn't be about specific knowledge of particular problems (although bad ones often are, usually having "Aha!" moments). They should test areas of knowledge: reversing a linked list is checking if they understand how pointers work, something which is not really possible to learn in 5 minutes of Googling, even if they can then answer the question. Questions should test general technical problem solving skills, which are not easily Googled, even if a specific instance is.

Finally, if a candidate requires knowledge of a particular API (e.g. the order of parameters to a function), then the interviewer should tell them. If the interviewer doesn't know, then they must acknowledge that it's not really important, and allow the candidate to just pseudocode it.