Innocent Spouse Relief provides you relief
from additional tax you owe if your spouse or former spouse failed
to report income, reported income improperly or claimed improper deductions
or credits.
Your father may have cashed in his 401K but in so doing he did nothing improper regarding his and your mother's tax returns. Your mother may consider filing a "married filing separate" return so to not have to include the 401K income but keep in mind, MFS taxes are calculated at a much higher rate.
