I was in the same situation a few weeks ago, the DVLA will charge you £160* and £200 surity fee, you get the surity fee back once you tax the car.
What was most annoying in my case was that I had it SORNd and on my drive for 2 weeks, I moved the car off the drive for 2 days so that the parents could empty the garage, I was caught and banged to rights.
By the time you get the car MOTd, insured and taxed it roughly covers the cost of the surity fee anyway, so if she doesn't want to use tax the car now it's a bit of a moot point as it's works out the same cost wise.
The best thing is, once you have all of the above settled the DVLA will write to you telling you that all of the above was in relation the the clamping agency, and that you now owe them £120 in fines as it is a public parking offence. Nice of them not to tell you this before hand!
Anyway, tell her to just realise she was in the wrong, as was I, and pay the fines. It stings a bit but what can you do?
Luckily, the car involved for me was my Civic VTI I made a post about on here, I decided to sell it and made more than enough profit on the car to cover the above fiasco.
*amount may be wrong, i'm a little worse for wear.