Hi,
Over the years I must have spent hundreds of hours in Schiphol Airport, and it remains my favourite airport in Europe to transit through. Strangely enough my wife is there at the moment, she's had a 10 hour transit and spent the time at the 'Yotel' which is an 'airside' hotel near the gates, she's on her way back to our home in the Middle East after a holiday in Spain.
Anyway, to answer your question Patrick, your wife will have absolutely no problems with a long transit like that, however she will be unable to leave the airport unless she obtains a Schengen Visa.
There are two hotels airside, one is called the Mercure and the other is called the Yotel, and there are actually very close to the 'D' gates for departures to the UK. To stay there would cost around 30 to 40 pounds, and they have showers, toiletries and free wifi. Otherwise you can also book a room in the Mercure for 15 euros for a quick shower.
Transitting in Amsterdam is generally easy, coming from an International flight to the UK it's all in one terminal, and it's much easier and simpler to transit then Heathrow or Gatwick. There are plenty of restaurants, shops and even a museum.
