Can you outwit the toughness granted by force field?
Is it just a "virtual" toughness as my friend says?

Can't outwit it but you can outwit the power that is necessary for it to work such as Barrier or TK.
This will then get rid of the toughness.

Sorry Stuart, but you're wrong.
Any power granted by Feats or Team Abilities can be Outwitted per the normal rules. Other than Force Field Toughness this includes the Outwit granted by Superman Enemy and the Probability Control granted by CSA, and possibly others as well.
Note though that Batman/Kabuki TA doesn't grant Stealth, it emulates Stealth. Therefore, it cannot be Outwitted.

You are correct however, that Outwitting the Prerequisite will also cancel the feat. However, in the instances where a figure has multiple prerequisites (i.e. Telekinesis and one of the other powers), it is perfectly legal to Outwit the Toughness directly.