If a wife was given a large sum of money that a husband made
($300,000) and she put it in her account.  Husband did not pay taxes
on the sum-- is the wife who files separate returns and owns her own
house solely liable for the sum?

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
estate.

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?