I find that hard to believe, as in if you get stopped by a police person, that they won't insist on some paper.
Hang on a sec - which country are we talking about here?
Has she lost her Polish or UK documents?
As far as the UK DNZ is right - there is no requirement to carry your car documents on you if you are in the UK. The police can easily check your insurance, tax and MOT at the roadside via their computer/radio.
If they suspect something is wrong or you don't show up on their database they usually issue you with a "producer" that requires you to produce your documents at a police station of your choice within 7 days.
If you have UK documents but are driving in Europe then you have to carry your documents with you at all times. Likewise for foreign vehicles in the UK.