CC's answer is correct because the law in England and Wales was changed to make it so, earlier this year. So that alone is a good enough reason for your Dad to make a Will (your Mum wouldn't get all the estate).
However none of this has a bearing on IHT, because there is no IHT to pay on transfers between spouses. Where IHT would potentially be payable is in legacies picked up by any children - using the current threshold of �325k (it is called the nil rate band) as the amount that could be left before IHT would becomes payable over this figure.
However the situation doesn't stop there. There is then another IHT scenario when your Mum passes away. The Chancellor changed the rules overnight in October 2007 such that when the second partner dies, the executors of the estate are able to use any unused nil rate band from the first partner (the %age unused), as well as the full nil rate band in force at the time of the death of the second partner. The two are added togther in assessing whether IHT is payable on the estate at the death of the second partner.
The long and the short of that means that if your parents net assets between them is worth more than �650k in todays money (2 lots of �325k), IHT will very likely be payable at the death of the second partner. Some IHT planning could be useful - you should be alerting your dad to what happens after the surviving partner passes away.