I got a real migraine at work once. I (obviously) couldn't drive home so I had to sit in a dark closet crying for several hours until it faded enough for me to go home. I think my boss knew I wasn't faking.
I know! Plus they happen a lot on Mondays and Fridays. But I know it's not me because this happened with their old boss too. I used to notice the frequency of the migraine thing and shake my head about it. Now I have to figure out a way to deal with it. Or not. Gah!
If it's often enough, you could ask for a doctor's excuse when they leave. It doesn't make for happy working relations, but it does start to set up a paper trail for future disciplinary action if they really do it enough. If you have chronic migraines, your doctor should and will know about it and probably will have given you treatment options .
If we call out sick too often here at school, the principal will request a note from the Dr. Now, with the HEPA laws you can't ask for specifics, but you can ask for official documentation. Might be enough trouble for them to think about it twice....