"Ordinary" native speakers always beat non-native speaker teachers hands down. If a teacher says one thing about a language and a native speaker disagrees, go with the native speaker.

Also, "acceptable" in such a context usually means that a native speaker might consider it grammatically correct and even use the form, but not all native speakers might agree. Or it may be acceptable in certain situations or at certain levels of speech but not in others.

I disagree with "by the evening I'll do the work" being acceptable. But I'm not a native speaker of English. However, "by the evening" refers to the point in time by which some activity will have ended - then at the latest, or at some point in time before that. Therefore future perfect is used. "I'll do the work" implies that work will begin at a specific point in time. Therefore I feel that the two phrases disagree, one states a point in time when something is over while the other implies it will begin.