The estate is going to be responsible. She needs to be very careful how this is handled.
First, and I say this as a general rule because states have variance in laws, the lien normally survives the probate of the estate. When the real estate is transferred it will come to her subject to the IRS lien.
Second, the IRS has priority for payment with the exception of the estate’s exemptions and administrative expenses. So she should not pay any creditors until the IRS tax debts are figured out. The IRS does not need to file a claim like other creditors, many people believe that the IRS has to file claims like other creditors and when it doesn’t they pay other creditors believing that the IRS is barred from getting paid. Big mistake.
Eric may correct me on this but I think that the tax debts (equal to 1/2 the equity) will survive due to the lien. The IRS (at least in Maine) seems asleep at the wheel regarding tax debts owed by the deceased. I almost think you could get away with murder here but never say never. Normally, I recommend to the PR to contact the IRS about the situation and what it can expect from the estate. See what the IRS does.