Probably the best way around it is to consult the actual player when using the ability so you know how to have Karna react. There are... a few special exceptions to this where this might not quite work (I won't say the name, please don't bring it up any further than this, etc), but for the majority of the players, I think that'd work.
As you said yourself though, it may not hurt to talk to Elf as the GM about this too, see what she has to say. An extra bit of caution every once in awhile never hurts, after all.