That's a great question! Let me break the answer down into parts for you. Like and as can generally be used interchangeably, depending on the variety of English being spoken. 'Like' is a more common phrase in American Modern English, however 'as' can be substituted, and it would make grammatical sense, but would be more akin to British or South African English varieties. For your first example, you can absolutely say "I ate like I never HAD before" since you want the tense of verbs to agree, since you're talking about things that occurred in the past. The phrase "I ate as I never have before" would be comprehensible, but since 'before' implies a past event, it would be more grammatically correct to say "I ate as I never had before." Your second phrases, "He treats me life I am one of his friends" is a perfectly normal, natural sounding sentence. "He treats as THOUGH I am one of his friends" would be a better option for the second. In the second phrasing, the 'as' could be understood as a way of saying 'since' or 'because', and would make that phrase seem incomplete or ambiguous. All that said, the difference is largely academic. However, as with all languages, English speakers tend to mix and muddle things up quite frequently. There are rules that dictate what phrasing is correct, but if you said ANY of those phrases, it would be comprehensible to a native speaker.