In over 30 years in the workplace I've met maybe two decent HR people. The rest have been nosey, power hungry, utterly unpleasant twats (and that's across a range of organisations, private and public). In my current job they're the nastiest I've ever encountered, which meant I've had to use my knowledge from previous jobs as employee and manager to make sure I was as protected as I could be from day one. I'm bloody glad I have that experience to draw on though and am not new to the workplace or the sector.
Firstly, is she in a union if that's an option? Thatcher pretty much defanged the unions which is one of the reasons employers walk all over workers these days, but it's still better to have union backing than not. If, God forbid, it does get to dismissal they can advise on constructive dismissal and help her fight her case at a tribunal if necessary.
Also, even the crappiest HR should allow her to have someone with her, depending on the meeting (not sure about performance reviews where your friend is). And she needs to keep records, in print or via personal email bccs if she can, of every conversation and email sent. I'm saying this as I know of cases where organisations have deliberately made it difficult to access systems when people are signed off sick. Also, copies of the company policies.
Secondly, does her workplace have a counselling service or Employee Assistance Program or similar and is she reassured that it's truly confidential? Unfortunately lack of confidentiality and tattle tale tits are a major factor in why people aren't using these services as much as they might.
Thirdly, does her workplace have an occupational health team she can ask to be referred to? This is what her crappy (and, I suspect, very inexperienced) boss and beyond incompetent sounding HR SHOULD have suggested. I'm just betting the boss has always enjoyed robust mental health and has no clue what anxiety, for instance, can do to a person. Although some other comments about needing to know the industry are possibly relevant, safety comes first, and that includes for your friend.
Fourthly, and she may want to do this first, has she been able to make an appointment to talk to her GP about getting signed off with stress? One of the very few good things to come out of COVID is that GPs are even more aware of mental health issues than they were previously.
And finally, what about women's aid charities if the root cause of this is the abusive marriage? Her citizens advice bureau should be able to suggest some suitable local or national helplines.
I am so sorry she's going through this and I hope a solution is possible.