it depends really.. if he aint insured on her car.. (as a provisional driver) then he is gonna have to pull his pants down
If he IS insured.. then it may just be a matter of failing to display L plates IF she has been driving 3yrs or more (she is 21 so its possible although unlikely)
for the L plate offence he will prob get a slap on the wrists and a £100 fine..
for the offence of IN10 then he will get fucked.. 6 points, £xxx fine...
She would also be likely to recieve upto 6 points and a fairly hefty fine.