I think it's likely a coworker. Why would a family member or friend bring up a past job in this way? This is the quote (pictured in a letter at the source linked in the OP) that makes me feel it's a coworker.
The quote contains the text exactly as written in the letter.
I just don't see a family member writing that, or even knowing enough intimate work details to have made the comment about backstabbing. I would first want to examine anyone she had an issue with at work who left the job before she did, followed by anyone she had an issue with at work who left after she did, or who is still employed there.
I would not call the ridiculous amount errors in the letter 'typos.'
Due to their large number, it is my opinion that the errors are intentional, or mostly intentional. Reasons the letter writer might do this include disguising the writing style of a person whose writing may be known to the recipient or disguising the intelligence level of the writer.
Interesting. I agree it seems like it might be a co-worker.
But I disagree it's someone faking illiteracy. The saying, "it's as hard to fake literacy as it is to fake illiteracy" holds true, IMHO. The writer of the letters is truly illiterate, IMHO. Interesting.