Correcting grammar when writing someone else's memoir?

Are you doing first person with the guy's narrative? His voice includes these mistakes, so I'd leave them in. Do make sure you punctuate perfectly, so only the words are incorrect while the writing mechanics remain flawless.

Start correcting those consistent errors, and you move on to better word choices (ne means badly, not bad), then writing it tighter, removing adverbs, phrases that don't add much, on to revising sentence construction, and pretty soon the voice becomes yours rather than his.

While that might make it a better read, it stops being his memoir.

Maryn, on a grey morning

I agree. Leave most "speech" irregularities in – but not so much in where it becomes satirical. You have to use your best judgment here.

And, of course, if you're concerned people might STILL think you have no sense of grammar, you can always put a short introduction (or afterward) that explains that you made a conscious decision to leave the garbled syntax. I did something similar in my last book -- my subject was a terrible speller, but I thought it was rather charming, so I left all the misspelled words intact -- and rather than insert tons of "sics" into the text, I just put a note in the Foreword that said I had made a decision to leave improper spelling intact when quoting my subject's letters.

I wouldn't correct the grammar because that's part of the character and you wouldn't be giving a complete portrait of the individual if you clean up the speech. I like Brian's idea of including this choice in the forward, although it's probably obvious.
In my memoir, when I'm upset I revert to more child-like language. I use shorter sentences and drag out some words, "Nooooo, I don't wanna." I'm not thrilled that I used to talk like that as an adult, but to change the sentence wouldn't accurately depict the situation.

POV

Dana, Are you using 1st person POV? I've recently been hired to write someone's memoir. I'm still trying to figure out the nuances. Any thoughts on the specific "how-to's" would be great!