I'm 18 and I've been on Nexium twice a day now for a couple months cause of severe reflux symptoms. I have noticed it makes me a little light headed after my second dose of the day but nothing I can't tolerate. Not sure why it does this, but I refuse to stop taking it right now as it is basically my life savior until they figure out what exactly is wrong.
Does her doctor have her take it after dinner and before bedtime? That's when my second dose of the day occurs and I find if I get myself ready for bed soon after, the light headedness isn't as bothersome. Maybe you can have her try this.