questions about Citibank account for parents

We opened a Citibank individual account
for my mother-in-law a year ago. Now Citibank notified
us that we need to either fill out a W-8BEN or a W-9
form. My mother-in-law has her own
ITIN (she doesn't have greencard and we have
not applied it for her yet).
We did put her as dependent for tax filing a couple
of times when she extended her B2 visa for stay over 6 months.


I know that W-9 is not very appropriate
since it is for a US person (including resident alien),
but the fact is that she has been claimed as dependent
on our tax return and we will pay tax on
the interest earned on her account together with ours.
Filling out W-8 sounds a lot more hassel
since we need to provide documentary to prove
that she is indeed a non-resident alien.

Does anyone have any experience/suggestions to share? Thanks a lot.