I have some neem leaf that I'd like to take as a powder but I read on here that neem oil can cause temporary infertility. Is this the case with the leaf as well?
Yes, the leaf also has this property.
I have a friend that has extremely high cholesterol (it's hereditary).
I personally do not believe high cholesterol is hereditary.
His dr. took him off of Niaspan on Saturday due to bad flushing side effects. He is still experiencing severe flushing each day even though he stopped taking it 5 days ago. Is this normal?
No. Flushing with the drug can last for several hours, but should not last days.
Is there anything he can do to prevent this?
That may be difficult. Normally when people experience this they recommend blood vessel constricting drugs such as ibuprofen, aspirin, or other nonsteroidal anti-inflammatory drugs. But these can create their own problems.
I would try a neurotransmitter inhibitor such as glycine to try and stop the signaling for blood vessel dilation.
How long will it stay in his system?
It should have been out of his system within a day.
Here is a post with safer alternatives: