You say fluids/magnesium, so I assume you mean an IV infusion. Do they ever give her anything else in the infusion besides fluids and magnesium? I also have intractable migraine. Sometimes I've had one that will go on for 6 weeks or two months (which sucks.) Usually, they just last five days to a week. The only thing that works for me is infusions. And when I'm in a really bad cycle, I have to get infusions multiple days in a row (I think the limit for safety at my infusion center for what I get is 3 daysin a row and then you have to take a break until you're a week from your start date, but your doctor can tell you more based on what your wife would be getting.) Anyway, I get depacon (valproate sodium), magnesium, fluids, and if it's bad, I also get DHE, zofran, and there's other stuff you can get like steroids and benadryl.
I don't know about your wife, whether she's going up and down, or it's just unrelenting and nothing is touching it, but my problem with a lot of the drugs they would try (like steroids, benadryl, triptans, painkillers) would be that they temporarily fix or improve the problem, but the rebound migraine they cause is even worse. But everyone is different. Her headache specialist would know best about her case and what would work best for you. I'm just throwing out ideas. I've read other clinics like Jefferson can give you infusions of other drugs (Katie Golden has written some great articles on it) that can help, too.
I also had to get myself completely out of any environment that could be providing triggers, which sucked. I'm guessing she's probably already doing that, though. Sorry this is happening.