Sorry, but I have to disagree. Just because he's broken the law doesn't mean it's ok for her to break it too. She can't reclaim possession of the flat until she's gone through the proper procedure. If she doesn't know tenant/landlord law then she shouldn't be letting out properties or she should get legal advice. If she felt the flat had been abandoned she should have left an abandonment notice on the door. If he wasnt paying rent then she needed to issue the correct notices and do it legally.
I'm afraid that the law will never change on this as it always favors the tenant. Appealing against the decision would be a waste of her time and money. She'd put the money to better use by paying for decent legal advice in future. The fact that he is a rapist and in jail doesnt even enter into the argument legally. To be honest, letting properties is a minefield if you dont know what youre doing and this is a fairly basic example - you dont enter the property until you have the legal right to do so.