I'm a native German speaker and i don't know of a dialect in Germany that would pronounce "Widerstand" with a short "i". Would you mind sharing which dialect you think of?

"ie" is always long. For "i" it depends on the splitting of the word, i think. I don't know if this is a concept in other languages too. I think the rule is that if the "i" is at a split, then it is long, but i'm not sure and there are always exceptions to every rule in German. Consider "Schnit|zel", "Lis|te" (short) vs "Bi|bel", "Wi|der|stand" (long).

I have worked for some time in Germany and this is how some coworkers pronounced it, but I do not know where were they from.

From what you say I assume that the literary pronunciation is also with long "i", which is good to know.

Perhaps they were influenced by the different spelling, because I have seen this phenomenon in other countries, where despite a mostly phonetic writing some words were spelled differently than pronounced, for etymological or other reasons, and then the pronunciation of those words by many people has shifted, matching the spelling and not the traditional pronunciation.