Yes to unions!
I went through a very bad stage in my career nearly 2 years ago and it actually started when I became pregnant and from comments said and details from other members of staff, it was pregnancy related discrimination. There were other issues too and I wasn't the only one with them.
My union were absolutely fab! In fact, they wanted to bring a case against the HT due to the evidence I had from conversations, observations and learning walks etc.
I chose not to. In fact, the union wanted to meet with the entire school and get everyone to bring a 'grievance' against the HT but it fizzled out to nothing and rumours were that people were understandably scared for their jobs; which is the reason why I didn't. Also I had hope that the matters could be resolved with a more positive outcome.
2 years down the line, I am still working in the same school albeit the relationship between myself and the HT has soured, feels false when we speak to each other and quite frankly, I have zero trust in anyone as I do believe that when the 'dust has settled' (it never will with me; too suspicious) they will put me on capabilities and it just so happens that I have a terrible class at the moment!
So I suggest, from guess work, that your friend needs to leave the school asap. If she is on maternity leave, you can seek employment from schools in the same county without having to pay back any mat pay and in some cases, some schools/LEAs over-look it. I am not sure how this will work if you are working in an academy.
It's a shame your friend is being used as a scapegoat. unfortunately teaching is like any other employment environment - subject to bullying, wrong-doing, discrimination that also goes on in the private sector. Sometimes rather than fight, it is better to simply move forward. I believe in karma! 