I have never had a migrain with out being wiped out the next day

Of course they can! What makes you think that people with migraines always have symptoms the next day? I would say the majority of people who suffer with migraines do not have any lingering symptoms the next day. I often have migraines so severe that I am in bed all day; even with migraine and pain meds. I also often have some nausea or queasiness with the migraine, but by the next day I am over it. Once in a while my head is somewhat sore or the queasiness might linger a bit but not often. I think it seems that you are looking to be critical of your co-worker and you don't know that much about migraines.

I wonder if the real question here has more to do with why does this person only call in sick on Monday or Friday and never on Tuesday, Wednesday or Thursday? Noone here answered that.

Don't know if Ebitda is a regular on this board, I myself am just coming back after a long summer break and more so b/c I'm really at a low point and looking for help. But a good question is being asked and Ebitda was put down. I think her real question is why the long weekends and why only when the coworker has sick time?

Not sure about Fridays, but Mondays are a little easier to answer. Messed up sleep schedules and maybe some drinking on the weekends can easily lead to migraines on Monday. So can eating different foods or different meal times. And family stress, out of the whole routine of the work week, etc. I've had many a migraine on Sundays just from the stress of Saturday with the whole family together. Maybe you could say that the anticipation of the weekend could lead to a Friday migraine, esp if there is a special event going on. Or maybe your town has Thursday night concerts, a club (book group, bunko, poker night, whatever), etc? that keep this person out too late and indulging. 

As for only using sick time, well..... if this person does not seem sick any other other day at all... I think you have your answer. I've only missed one day of work from a migraine, but I work PT. On my days off, I sleep if my head hurts and I'm nauseated. Otherwise I suck it up and take meds. I've been in the bathroom on the floor in tears before when I had less control, and just last week I was late for work, kept a trashcan next to me and a hand on my forehead while I typed to sheild my eyes. My coworkers left me alone. But the glory of medication made me functional by lunchtime. And I was a whole different person by the end of the day. I don't get paid if I don't work, so I make my best effort to get there.

So while there may be real migraines and you could be on the wrong track, I personally think it's odd that they only occur on Monday or Friday too. But then again, there could be logical reasons.