I would approach her on the rumours. I would say if she has a valid certificate that there isn't much you could do. However i would think a good detailed reference from you would be more important than taking those days off to get her 'entitled' sick leave. Although you apparently can't write anything bad in a reference you don't have to write anything too detailed especially if she has let you and herself down in that last week... the old saying goes "if you can't say anything nice don't say anything at all'.. On the other hand it may be better for you to not have her there anyway if thats her attitude, she may bring everyone else down...another old saying applies here too "good riddance to bad ..." enjoy