Your friend is wrong and is likely damaging his engine rather than protecting it.
Letting your car idle does little except create negative conditons internally. Now, if he wanted to take it out for a half hour run every couple of weeks, that would be good.
Personally, I have never seen any internal rust on an engine that was functioning properly. On the other hand, it it were stored outside at the beach, I guess it is possible.
WD40 works well on rotors in storage.
Spray it on and forget it.
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