Follow this rule whatever the final consonant. Thus write,
Charles’s friend
Burns’s poems
the witch’s malice
This is the usage of the United States Government Printing Office and of the Oxford University Press.
Exceptions are the possessive 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.