How long does it last? This has happened to me a few times, but aside from the fact that my roommate would say it's because I'm blonde :)...When I work on deep breathing...it goes away. Is she a "chest breather"? Does she take short breaths or long ones? This type of thing alone can make a big difference.
When my clients get dizzy (I'm a personal trainer & posture alignment and pain management specialist) I have them put their arms overhead. This changes blood flow so that it goes to your head faster when you're standing up or sitting down. That's why they tell people to put their head down when they're dizzy besides not wanting them to fall over.
If this continues to be an ongoing thing...I don't mean to alarm you, but I would take her to a heart doctor. Sometimes, when the heart isn't pumping enough blood, it can cause dizziness and fatigue. One of the gals at the gym has a family acquaintence who was in surgery yesterday for exactly the same thing. It took them a long time to figure out why he was so dizzzy all the time, but thank God they did.
I DONT intend to alarm you but if this is NOT a happenstance issue and is ongoing....you need to take action. Hope things are resolved soon.