While they often work overtime, their base work week is still Monday - Friday (5 days). Therefore each working day is 20% of the total working days. It stands to reason then that with a normal random distribution of sick days, 20% will fall on Monday, 20% will fall on Tuesday...20% fall on Friday. Thus 40% fall on Monday and Friday.
So the boss it outraged that, in fact, his employee's sick days are randomly distributed (as should be the case) and are not unnaturally frequent in conjunction with weekends (as would be expected if they were faking).