The complement of the class of friends is not the group consisting of enemies, but rather the group consisting of non-friends. For terms consisting of more than a single word, things are a bit more complicated. This is not technically correct, but it is acceptable so long as there is an implicit understanding that our discourse is suitably limited in this case, limited to talk about animals. All dogs are mammals. No reptiles are humans.
All dogs are mammals.
No reptiles are humans. All lonely people are prone to exploitation by others. Certain species of whales are in danger of extinction. Some court procedures are so complicated as to be very inefficient. No Russian authors are insensitive to nature.