We can sufficiently enforce auditable and reviewable restricted passkey access using comments and naming, without overloading the C++ type system's "friend" feature.

The resultant designs are simpler and more clear.
Using friendship restrictions is imprecise and requires a lot more documentation, and forward declarations etc of foreign entities. It should not be done.

See
https://github.com/10gen/mongo/pull/55467 (fixed in review)
https://github.com/10gen/mongo/pull/55632 (needs fix)

Happy to discuss or collaborate on alternatives.