I couldn't find wording that makes it invalid to say friend virtual... The closest seems to be 9.2.3 [dcl.fct.spec] paragraph 5, which says:

The virtual specifier shall only be used in declarations of nonstatic class member functions that appear within a member-specification of a class definition; see 11.7.3 [class.virtual].

I don't think that excludes a friend declaration (which is a valid member-specification by 11.4 [class.mem]).

John Spicer: I agree that virtual should not be allowed on friend declarations. I think the wording in 9.2.3 [dcl.fct.spec] is intended to be the declaration of a function within its class, although I think the wording should be improved to make it clearer.