Chapotraphouse

Most sinitic languages use the same basic script plus or minus some words for vocabulary differences because it's a logographic script.

That's what I originally thought, but technically every single major Chinese topalect has their own unique set of characters. I'm not sure if Taishanese has its own script, but I know Cantonese does, which isn't the same as the vast majority of Taishanese people would remind you. And in any case, written Cantonese is pretty different from written Mandarin even with simple sentences but it's not just the difference in characters but the fact that the characters that they do share with each other have diverged in meaning since Middle Chinese. 食 is a root that means "food" or used in words like "edible" (食用) in Mandarin, but just means "eat" in Cantonese. There are also grammatical differences that affect order of words. And this is all considering that he spoke Taishanese not Cantonese.

At best, he would've learned the Mandarin character set but written them out with Taishanese/Cantonese character order and substituted those Mandarin characters with a more appropriate Taishanese/Cantonese character when saying them out loud, meaning that he essentially would have spoken Taishanese but written with some bizarre Mandarin/Cantonese hybrid. So, writing "吃" like standard Mandarin but saying "食" since he doesn't know Mandarin.

I also should have elaborated that I'm extremely skeptical that a 6 year old member of the Taishanese diaspora living in the US during the 80s would've been exposed to the Taishanese/Cantonese character set. It's mostly based on my experience with Cantonese people. I largely associate people typing with Cantonese characters with young people. Most older people just write in that Mandarin/Cantonese hybrid. Maybe that's a misconception on my part.