Well, Lynne Truss (the author of Eats, Shoots & Leaves) is very British, and she states that it should be "Remus's lover"; however, she also acknowledges that this is one area of apostrophe usage in which one cannot be too militant as there are militants arguing the virtues of "Remus' lover" with equal passion.

So, let's all just agree to gang up on the fools who stick apostrophes into words which are plural but not possesive, or who forget to include them altogether!

P.S. But I ask all of you who prefer "Remus's lover" and "James' balls," don't you hear the second s? I do.