I bought my wife a iTunes gift card today and she has £15 on her account but she has just tried to buy a game and for some reason it's asking her for her card details but she already they have them details on her account.

So if she is trying to buy it from her £15 then why is it asking for her card details?

It might just want a security check where you have to put in the 3 digit security code. I think it does this from time to time.

Sometimes they just want to verify your account and personal information. It doesn't ask like this every time.

If the account has been accessed from a new device this is usually why