Well in the first instance it probably depends on whether Revenue know that your parents were "married but living apart" - that is the box that either or both of them may have ticked on any correspondence with Revenue in the past 30 years.
In any event, she should just do things right.
Did she own half the house with your Dad all along, in which case she only inherited the other half last year? If so then she will have inherited that half at its market value at the time, so would effectively only be liable to CGT on the other half of the house that she had owned all along, less PPR relief for the period from 1979-83 plus the last 12 months.
So the absolute maximum she could owe in CGT on a sale should be around 10% of the sales price. Surely worth it for the peace of mind of knowing that you don't have a ticking tax timebomb.