A friend told me recently that she was bullied into approving an ethics application for a student who wants to do trans identity research with primary school children with no parental consent involved…
This is horrendous. I think your friend should look up the university's whistle-blowing policy. There should be someone at the governance level (i.e. removed from the day-to-day running of the university, and probably a lay member of the governing body) who is designated as the recipient of whistle-blowing concerns and who can deal with them on an anonymous basis.
I'm sure your friend has put her concerns in writing and is keeping them. That'll protect her to some extent if there are subsequent complaints. But this is very serious because it involves the safeguarding of children, so as a responsible professional she may well be considered to be in breach of duty if she doesn't see her concerns through.
As you say, this is far from easy, and your friend should be able to go to her union and know that they'll have her back. Not only should they support her, they should negotiate better practices locally to ensure no-one else is put in that position. Beyond infuriating that UCU is more likely to harass than support her.