That's just bad grammar. Although the native speaker could be poorly educated, it is more likely that he was just being lazy. Good grammar requires thinking, and some people don't put in the effort.

The use of "would've" with past participles like "saw" or "did" instead of "seen" or "done" is a grammatical error known as a "hypercorrection." This error occurs when someone tries to apply a grammatical rule too rigidly and ends up using the wrong form of a word. For example, in standard English, "would've" (a contraction of "would have") should be followed by the past participle form of the verb, such as "seen" or "done." However, in some dialects or informal speech, people might use regional or colloquial variations, leading to phrases like "would've saw" instead of "would've seen" or "would've did" instead of "would've done." It's possible that the native speaker you encountered was using a regional or colloquial variation, or they simply made a mistake. In any case, it's not considered standard or correct usage in formal written English.