I take 10 mg of melatonin and 4 mg periactin (cyproheptadine), which is an antihistamine. My doctor said it was similar to benadryl and I'm pretty sure you can buy it over the counter, although I am prescribed to it. For some reason the combination knocks me out for a solid 10-12 hours. And it's great, restorative sleep. I actually feel rested upon waking up. Melatonin alone didn't work for me, nor did ambien or benadryl.

Not sure if this was just luck for me or if it will be beneficial to your friend, but I definitely recommend giving it a try.