I REALLY feel for your mother-in-law. I also suffer from migraines, since I was in 4th grade and i'm 24 now. However, I didn't get diginosed with them until about 2 years ago.
One of my main triggers was aspertane, which is in almost every diet soda or anything sugar-free that is not made with splenda. I was getting migraines everyday for a rather long time. Once I stopped consuming aspertane, my migraines went down a lot.
When I was getting them all the time, the doctor had me try several medicians, some that worked but made me feel so horrible that i'd rather deal with the migraine, and some of the ones I tried didn't do anything. Tell your mother-in-law that if the medician's arn't working, to tell her doctor that she wants to get off of them. What's the point in staying on the meds if they arn't working? I know one of the medician's my doctor put me on was for high blood-pressure, not because I have high blood-pressure, but to help prevent them. I didn't really like the way I felt on it so when my migraines calmed down after stoping the aspertame, I told my doctor that I wanted to be taken off of it.
However, i've had a migraine now on and off for 2 weeks. Nothing seems to really be making this one go away. I've been back to the doctor and now i'm on some kind of anit-depressent medician that is suppose to be a preventiave medician. The doctor also gave me a presricption for a pain killer, which hasn't even touched it. It made it so that I didn't care my head hurt, but it still hurt. The best combo of medician that i've found that USUALLY works for me is 2 excedrine migraine and 2 advil migraine, although that hasn't done anything for this one.
My mom suggested that I try making a tea out of dried thisle(sp?), that took away her migraines back when she use to have them. I haven't tried this yet, as I havn't really felt like going out more then I must.
I hope this gives you more ideas and again, tell her to talk to her doctor about coming off of the medican if it's not working so she dosen't keep throwing away all her money.