One becomes
eligible to apply for citizenship
once (s)he is a lawful permanent
resident for five years (but that
is generally reduced to three for
LPR's married to a US citizen).
The problem is that the clock doesn't
even start until permanent resident
status is granted, and none of the
time prior to that point counts.
I'm hesitant to suggest anything
beyond consulting with an immigration
lawyer, in part because I don't
know all of the facts (or for that
matter, the law) but in theory,
your son should be able to sponsor
her for permanent resident status
as a direct relative, and once that
petition was approved, her three-year
clock would then start.
