She would not be personally liable for any income taxes on the money
if she did not file a joint return that year. However, the IRS would
probably take the view that the husband's gift was an attempt to evade
taxes & still protect the money from IRS levy. Therefore, she would be
liable to the extent she received funds ($300,000). This is similar to
the way a beneficiary of a will is not liable for the decedents's
debts, except to the extent the beneficiary received assets from the
If the IRS prevailed, a court could make her pay the taxes & penalties
& interest & penalties on interest, etc, up to $300,000 total. Note
that this applies even if she's spent/given away the cash. In that
case, the IRS could get $300,000 of her other assets.
What was the source of the money? Earnings, capital gains, inheritance?