My friend lives in Canada and is a Canadian citizen. How long can she stay in the US and does she have to return to Canada for a certain period before returning to the US legally ?

Canadian citizens are allowed to enter the US as visitors, generally for 3-6 months.

Yup, 6 month maximum each time she enters, Canadian citizens are normally not given any kind of visa unless border patrol believes they will overstay the legal limit so they don't keep track of how long between visits. Therefore going home for a few days between stays is okay unless she gets issued a visa that must be returned when she leaves the US

Its not a visa that is turned in to CBP, it is a form I-94. Completely different. A visa is a machine readable laminated document (MRV) in your passport. It has your picture and several security features. This is not torn out of your passport and submitted to the CBP officer at the POE.

It's actually not "completely different" a visa is a document with a specific date you may enter and date you must exit the US.

The I-94 is basically exactly the same thing, just not permanently affixed to your passport.

For the sake of clarity I said "visa" but mean that unless she is given any sort of official document that must be returned at departure then no one can really track her movements in and out of the US.

It's actually not "completely different" a visa is a document with a specific date you may enter and date you must exit the US.

The I-94 is basically exactly the same thing, just not permanently affixed to your passport.

For the sake of clarity I said "visa" but mean that unless she is given any sort of official document that must be returned at departure then no one can really track her movements in and out of the US.

The visa and I-94 are completely different documents. They serve completely different purposes, issued by completely different government agencies (and not sub-agencies of the same parent agency), have completely different validity dates, expiration dates, completely different security features (or lack therof) and are not "basically exactly the same thing."

Visa: applied at AMCON or US Embassy in foreign country
I-94: applied at US port of entry

Visa: application on Form DS-156 or DS-160
I-94: application using the visa issued

Visa: MRV fee and visa reciprocity fees applicable, varies by country
I-94: generally no fee, does not vary by country

Visa: multiple US/intl security checks, including visas Donkey, Eagle, MANTIS and TAL
I-94: none

Visa: laminated with photo, bar code, machine readable strip and multiple security features
I-94: printed, index card like form, filled out by applicant

Visa: document allowing application for entry to US in a specific NIV classification
I-94: document controlling NIV status and duration while in the US.

Visa: replacement if lost, in-person application AMCON or US Embassy in foreign country
I-94: replacement if lost, I-102 at USCIS Service Center

Visa: issuance controlled by Foreign Affairs Manual (FAM) and 22 CFR
I-94: issuance controlled by Code of Federal Regulations title 8 (8 CFR)

For sake of accuracy and therefore clarity, I said the documents were completey different because they are completely different.

If none of the legal explanations is sufficient, know this: visa issuance can take months, with multiple requests for evidence and a sworn personal interview with a US Department of State, Consular Officer--the I-94 is handed out by your flight attendant.

Respectfully, they are completely different documents. You can confirm all of this with any experienced board certified immigration attorney or US immigration law professor.

I worked at the US/Canada border for Canada Customs and Immigration for 5 years if you've ever read my bio, hence why someone decided to consider me an expert in the field.

In layman's terms they are very similar, both documents are made to track someone from a foreign country into and out of the US and ensure they enter and leave when required.

I'll be more specific in the future if required (technically I said "some sort of visa" as someone who knows nothing of immigration law might understand this rather than "she might need an I-94"). I didn't feel it was necessary to bring all this up as the only question asked was how long can someone stay in the US from Canada legally and not which of these 2 documents would be required in this case.

I worked at the US/Canada border for Canada Customs and Immigration for 5 years if you've ever read my bio, hence why someone decided to consider me an expert in the field.

In layman's terms they are very similar, both documents are made to track someone from a foreign country into and out of the US and ensure they enter and leave when required.

I'll be more specific in the future if required (technically i said "some sort of visa" as someone who knows nothing of immigration law might understand this rather than "she might need an I-94"). I didnt feel it was necesary to bring all this up as the only question asked was how long can someone stay in the US from Canada legally and not which of these 2 documents would be required in this case.

I didn't bring it up. I only answered the question directly. :)