Sorry but the more you post the more I'm not surprised they're managing him out.
That there was a grievance even MADE in his first year is very concerning, that it wasn't upheld isn't necessarily something to be proud of.
PLUS a poor review, AND then your post at 1126 yesterday rather suggests
A - it's not the job he enjoys but what it brings/offers
B - he's a clock watcher, never a good thing. (Senior roles of the type filled by head hunters, paying good salaries naturally expect employees to go above & beyond - that's always the case. I wouldn't expect anyone but entry level staff to be getting away at 5-6pm every day! And that's mainly because they're usually on a wage not a salary and so aren't encouraged to work extra lest they [rightly] expect overtime pay. And I've worked in a wide variety of roles, levels & industries since the 80's until a few years ago when my own health prevented work).
All those combined with your comments about it being a blame filled horrible industry/company...
Sorry it sounds more to me like neither of you are realistic about how employment works, in the uk (are you both British? I think you're living & working in Britain?) particularly in the current climate.
You can AS me if you like and you'll find I'm quite left wing and highly critical of employers not treating employees well but I'm not convinced that's the case here, certainly not totally.
Did he really take on board the issues brought to his attention by the grievance and the review and address them? Or did he just complain it wasn't fair and everyone hates him?
Has he done anything different to show his commitment to the company and the job? Has he sought to improve his approach?
At this point as well as looking for another job (and not being too fussy about that, it's an employers market at most levels right now, add to that this sounds like an industry where word gets about, so the sooner he's employed elsewhere the better before his reputation truly precedes him. He also needs to consider they'll not want his notice period to fall past the 2 year mark), he needs to honestly consider how & how much he's contributed to this situation and how he can avoid it happening again.
"not sure what your beef is but comments like “strolling out at 5:30” are very outdated" no it's not, I know very few people who get to leave work bang on time every day, at all levels, without it would likely have a very negative effect on how they were viewed by their boss/company.
"You both need to be realistic about what real companies expect of senior managers (and less senior ones for that matter)" totally agree.
"He didn’t ask for training but coaching" splitting hairs tbh. They clearly expected him to be able to perform on certain tasks and then he's not done so.
"Yes you should be reaching your targets of the job you’re hired to do but if you underperform in an area I think it’s ok to ask the business to support you in tackling this..."
Depends on the reasons why he's underperforming.
It does rather sound like he's just out of his depth, they're partly responsible for not realising this during the recruitment process and now it's all very awkward for everyone.
But he needs to be honest with himself in acknowledging that and correctly assessing his skills and aptitudes in being realistic about what roles and what level he can work to. That may mean a lower grade/salary.
But you both need to be much more realistic about working hours/conditions these days.