Maybe you could ask her and come to some kind of arrangement. Tell her you know of her plans and offer her a percentage if she doesn't take the week off. In a round-a-bout way suggest that this could affect any references too and you will mention it to any potential new employer.

Seeing as she has been good enough to not use her sick leave for the fun of it (as many many do) she's probably frustrated that it's going to go to waste. I have been in the same boat and my DH walked away from about $35,000 in accumulated sick pay and said he felt like an idiot for always doing the right thing by his former boss!