I have had this problem too. It started after I lost a bunch of weight in the beginning of the year, and I naturally have very low blood pressure. I started fainting and getting seriously lightheaded every time I stood up. My cardiologist suggested staying hydrated, which made a big difference.
Then, after I went Paleo, it started happening again. I consulted with a nutritionist, who checked out my food logs. He suggested a fat imbalance is often a cause of postural hypotension. I have upped my fat intake with the "healthy" stuff - olive oil, almonds, etc, - and it has also made a difference.
Hope that helps!
When you eat low carb, especially at the beginning, it tends to have a diuretic effect. This, in effect, lowers the blood volume, which then lowers the blood pressure. When the blood pressure is low, it takes a few minutes for your body to "catch up" when rising from a sitting or lying position. Note to original poster: this is not a "nutritional deficiency" :-)
Have your friend add a little salt to his foods, and have him rise a little slower. His body will soon grow accustomed to the reduced blood pressure and it will learn to compensate for the lowered blood volume. This is not a dangerous condition.
NOTE: I am not a doctor. It's entirely possible he's dying of some strange malady of which I know nothing, but the people I know who have gone low carb (including myself) have this exact situation occur almost without exception.
I've been having a similar problem. I am always on the low blood pressure side, but recently it got worse... I am drinking a lot (could it be too much?), not counting calories, eating meat with fat and adding fat whenever I can.
I think I will use the suggestion of using more salt... I am eating carbs (even if pretty low, mostly from cream/cheese and veggies) so I am pretty sure it can't be too low carbs.
Have your friend had a chance to check his blood pressure at the moment? Could the weather be an issue? I know I feel much worse when there is a serious heat wave. Could it be over-training on cardio-like exercising? That's what hits me sometimes.
I would advise more observation - checking daily blood pressure at set times plus at moments of particular light-headedness and look for patterns.
Possibly dehydrated. Lack of water can cause this.
I used to get that when not eating enough fat. It can also be a problem if not enough food is eaten, but there may be something else happening. How long has he been low-carb?
Great suggestions so far. Also consider:
1) Iodine deficiency leading to hypothyroidism, or