Follow this rule whatever the final consonant. Thus write,
Charles’s friend, Burns’s poems, the witch’s malice, the Beatles’s concert tour
This is the usage of the United States Government Printing Office and of the Oxford University Press.
Exceptions are the possessives of ancient proper names in – es and -is, the possessive Jesus’, and such forms as for conscience’ sake, for righteousness’ sake. But such forms as Achilles’ heel, Moses’ laws, Isis’ temple are commonly replaced by
the heel of Achilles the laws of Moses, the temple of Isis
The pronominal possessives hers, its, theirs, yours, and oneself have no apostrophe.
