headaches after long run?

How much did you drink? Have you done a sweat test?

Typically headache after long training session or race, means you are dehydrated. I used to get them after my races. I've worked hard to learn to drink more while training and racing.

This might be an "exersional headache".  Occasionally, when I do something with a really high level of exersion, I'll get a bad headache that feels much like a hangover.  The pain patterns run on the side of my head, at temple level, and run front to back.  Rest is the best thing for it to go away.  I can be more susceptible to this pain pattern from any physical activity for several weeks.

Maybe do a google search on exersional headache.  See if that might be what you are experiencing.

I used to get headaches a lot in the winter as well when I did long runs.  I was doing the same thing and not drinking much thinking that sense it was cold I wouldn't get dehydrated.  Once I started carrying water with me and drinking it on a regular basis while running, my headaches have gone away.  Try and bring water in a fuel belt or a handheld water bottle on your next run and drink it regularly. 

I get bad headache/nausea/fatigue spells after long workouts if I don't take in enough electrolytes. About 20 or 30 minutes after the workout, I was usually shot for the rest of day. The only thing that would make them go away was sleeping.

I'd tried dealing with it by adjusting fluid intake, but no amount of hydration helped (and extra water made it seem worse). I was seriously thinking I'd have to put a time/sweat limit on my workouts.

Finally, after noticing that I didn't feel quite as cruddy after using Gatorade Endurance, I got some Salt Sticks and it was like flipping a light switch. I could go on a 3 or 4 hour ride in hot weather and other than being tired from the exercise, all the other effects just went away.

You'll see a lot of people say that suppliments aren't necessary and I agree that is probably the case for most people. However, if I've forgotten the Salt Sticks a couple of times since I started using them and sure enough, on came the headaches (taking them after the headache helps a bit, but not nearly as much as taking them proactively).

If it's every time, I am hesitant to say that it's dehydration, unless you are chronically not drinking enough.  But then there would probably be other issues, too.

Could be tension in your neck/shoulders, especially if it happens in colder weather.  Some people tend to scrunch up more when it's cold and windy out, and running all tight like that can cause headaches.

Why is the first thought dehydration?

Could be she takes in less caffeine than normal on long run days.  Could be muscular tension.  Could be weather-related.  Could be brought on by exertion.

Dehydration takes a long time to occur.  If you mean an electrolyte imbalance, that's different than dehydration.  She could be hyponatremic, as well, which is the exact opposite.