It is very difficult and usually only successful when you're a celebrity or large business owner etc. It's not enough that someones lies about you - you have to show you've lost something that gives rise to damages. In my opinion Fiona has not. She does not have work or a livelihood to lose. She is not a public figure who might lose work such as Johnny Depp. As has also been pointed out over and over again, neither Gadd nor Netflix have said Fiona is Martha, even now. Truth is a defence to defamation, anything about stalking I expect will be covered under that. People keep saying it's not enough that Netflix said 'it's a true story' with some other statement at the end - without being a media lawyer, I would really expect that the Netflix legal team either knows enough about media law to know they've sufficiently covered themselves, OR they have already built a realistic quantum settlement into the budget for the show, on the basis that this has all riled up a lot of attention for the platform.
The only other thing I would say is look carefully at who is saying they will take on Fiona's case or that she has a case. Look at what kind of work Chris Daw KC does, look at the fact Piers Morgan keeps asking American lawyers out of our jurisdiction to comment, look VERY carefully at how the statements from lawyers in this jurisdiction are framed - in my opinion, they all want the public to think she has a case, but if you read between the lines, it's a different story. Not helped by people going 'what about duty of care' (duty of care has limits, it doesn't apply to everyone you'd think it might and it has a strict test to determine where it does apply or not) and 'what about her reputation' (again means something different legally than you'd think).