To be honest if she's not breaking the law they can park where they like, you could "have a word" but it's as likely to cause bad feeling than be OK, depends on what kind of relationship you have with your neighbour.
I used to park one of my cars on the public highway every now and again, happened to be outside a high end kitchen shop, the old fella came out one day and starting "throwing his weight around", parked it there all the time after that. If he'd have asked me nicely I would have considered it, but being a belligerent **** seemed more his style so as far as I was concerned it was too bad after that!