As the link provided by Michael says, the threshold is £325,000 - but as we don't know the circumstances of your friend, I'll add the following:
"Married couples and civil partners are allowed to pass their possessions and assets to each other tax-free and, since October 2007, the surviving partner is now allowed to use both tax-free allowances (providing one wasn’t used at the first death).
At the extreme, this effectively doubles the amount the surviving partner can leave behind tax-free, without the need for special tax planning."