This is from the Dutch Royal Family Official Web Page
On 20 June 2003, the Prince became engaged to Mabel Wisse Smit. The wedding took place in Delft on 24 April 2004.
Through his marriage he is no longer a member of the Royal House. Under the Membership of the Royal House Act (2002), he no longer bears the title Prince of the Netherlands. He still bears the personal title of Prince of Orange-Nassau, and will continue to be addressed as “Your Royal Highness”. The hereditary title Count of Orange-Nassau has also been bestowed on him.
Any children born to Prince Friso and Princess Mabel may use the title of Count or Countess of Orange-Nassau, Jonkheer or Jonkvrouw van Amsberg, and the surname Van Oranje-Nassau van Amsberg. Should they do so, they will be referred to as Count or Countess. Prince Friso’s wife will not become a princess in her own right through her marriage. However, custom allows a wife to use her husband’s titles.