J.B.
It depends on how large she intends the account to be and what she intends to save it for - is she thinking of it as a high school graduation gift or college? Are we talking a few hundred dollars or thousands?
UGMA or UTMA accounts are considered the assets of the child so if the account has assets in it when you file the FAFSA in the fall of senior year, 20% of the account balance will be counted in the EFC (expected family contribution). A solution to that would be to spend down the account before filing the FAFSA, perhaps buying a computer, a car, paying for a vacation or some other large expense.
A regular 529 that the grandparent holds is considered an asset of the grandparent and is therefore not reportable on the FAFSA. However, any money distributed from that account is considered untaxed income of the student in the year it is distributed, so while that might be a great idea for Freshman year, if 529 assets are actually used to pay tuition that year, it will severely increase the student's EFC for the next year, and so on.
A custodial 529 is treated like a parent asset, which is better than having distributions count as income or have the money count as a student asset as it would in an UGMA or UTMA.
If the goal is college savings, a custodial 529 might make the most sense. But if the goal is to give her a little spending money to fund some of those more extravagant things that teens like to do (a laptop for college, travel abroad, attend specialty summer camps, buy a used car) then an UGMA account that is spent down before the FAFSA application is completed might be the right option.