You can go with the right way, or the wrong way.
Right way: Stomach flu, food poisoning, or allergic reaction to seafood. None of these need the care of a doctor, so it's assumed that no doctors note will be provided, and will be ok.
wrong way: Deaths in the family, doctor appointment, women problems. These are all traceable, and will come back and bite her in the ass.
You need something untracable, that will only need one day, and doesn't have any after-effects (like a cold, you can usually tell when the symptoms are showing up, unless you do it immediatly following a weekend)