Yeah, you're right. Well we were very kind to her when explaining that she should leave, and I'm sure that right now she's betting on winning my wife back by playing the victim or something (she might succeed to some extent, I'm afraid, cause my wife is sad for losing her friend, and regrets that all of this happened, etc.)
According to "In Sheep's Clothing", one should try to put things in such a way that the covert-aggressives think they are the ones who are winning out of a split, cause the intolerable thing for them would be to feel 'losers'. So I have asked my wife to remind L of all the things she actually got from us, both straight and maybe not so straight (like borrowing money and 'forgetting' to pay it back, and perhaps even stealing from us, which is quite likely), and to openly state that in the final balance she is pretty much the 'winner' in here. Of course she will reply that she is not competing cause she loves us and blah blah, but hopefully the message will get accross, she will go with a smile and not think of taking some sort of revenge. Maybe adding that IF she does, we'll take more serious measures (i.e. police) might also help.
Thanks a lot for your thoughts, anart.