I have noticed that native speakers confuse it sometimes.
Is this ok in an informal coversation to say like "She have <something>" or "Do this item look fine?"?
No...and as a native speaker the only time I've heard something akin to "she have...." would be among uneducated speakers...not something students learning English want to strive for.
What about "He don't look right too me"? I think it's acceptable to say that in a tongue-in-cheek way.
"He/she don't" is sometimes used in songs.
"He/She don't...." is used in ebonics and thus rock 'n roll. Yeah, it can be used in a "tongue in cheek" way, but it's definitely not what we'd call proper English.
However, different rules apply for writing good song lyrics than those that apply for writing good essays.
"He don't know nothing"
as opposed to the grammatically correct
"He doesn't know anything."
Double negitive is sometimes used idiomatically
for emphatic emphasis.
I wouldn't use it unless I was familiar with that idiom.
Regards, Paul V.
Yes in some people's speech - none of which seems to be acceptable outside those who speak it - double negatives are used. Double negatives are not used in conventional English but are used in many other languages (e.g. French) so maybe there's an influence from non English languages.
In English, it IS possible to say "she have."
You say "Would she have a cat?"
Or even "Does she have a cat?"
"She have" is common in English.