With respect, I think you may have only been given part of the story by your friend, or she has exaggerated a bit. There's no way one letter would cost £1000 - and, even if some solicitor did quote this, why would anyone pay that amount?
And, why did they both move out, presumably leaving the house empty? If she's paying the mortgage, surely it would be better to live there?
I think I'd stay out of it, apart from polite enquiry, as you're only going to get one side of the story anyway and, as you say, you don't actually know her that well anymore.