I have no idea; however,...
the late Carlton Fredericks had an "anti-grey hair formula" comprising: copper, cobalt, PABA and pantothenic acid.
PABA and pantothenic acid are two B-vitamins available in the U.S. over the counter.
"Cobalt" equates to Vitamin B12.
Be careful about copper: it's sold, in the U.S., over-the-counter; however, supplementation can upset the important copper-zinc balance.
Your friend might try low dose supplementation with these four and see how things work out.
One other possibility is low stomach acid, which reduces the girl's ability to absorb these and other nutrients from her food. Some U.S. doctors oriented to nutritional treatments can test for this. I had the test done some years ago with a "Heidelberg capsule." Digestive juices, along with other things, tend to weaken as we age reducing our ability to absorb nutrients from food.