It can happen when wake from sleep as during sleep blood pressure is usually very low and sudden standing and walking might not instantly trigger nervous system to raise bp or raise heart rate (to reach blood to brain). if both of these won't kick in fast lightheadness and even fainting can occur. Fainting is body's natural way to reach more blood to brain as when lying flat on ground heart and brain on same level.
There is tip to reduce it. Use a hard pillow during sleep thus raising head above always.
Along with what the other member posted about low blood pressure if she is anemic from heavy periods that could cause a person to pass out. So ask her if she has super heavy periods as well encourage her to go for a check up. A simple blood test can tell a lot. Best to be checked by a doctor.
Does she have hypotension (low blood pressure)? That can cause blackouts at times, but especially on just waking up (when our blood pressure tends to be at its lowest). Has she been diagnosed with some other medical condition?
Being aware of having low blood pressure can help (eg. by paying attention to how faint we feel so we know when we should be moving cautiously, taking a deep breath at times to help boost our heart rate and blood pressure so we're much less likely to blackout).