I think the poster is saying the way the op posts could be indicative of the fact she struggles with written communication. Possibly not, but considering thays what her pip is about, and some of the reason behind hee poor performance reviews, it could be an indicator there is an issue there.
however it is fairly irrelevant, the company can have a business case to deny flexible working, it is not automatically discriminatory to do so, she needs to provide further information on whether this is a company wide policy etc,
however the key point here is the company can say something as simple as they have a business case to deny, and feel work quality and performance falls if she works from home, for her, they can then show the poor performance appraisals, which is documented evidence.
I do think they are moving to termination due to performance, and I think if I was the op I’d be focusing on that, and not flexible working right now. If her errors in her written work are an issue and she’s on a pip there is no excuse for her boss having to repeatedly point out more errors, they should not be occurring.
the comments she has said he made are discriminatory, but unless there were witnesses, then it will be her word against his, he could deny it, and it could look like a poor employee making stuff up to try to force flexible working.
its unfortunate the flexible working issue is at the same time as the poor performance issue has come to a peak. But as said, if I was her id be focusing on perfoming well if she wants to save her job, which it seems she does.