My sister-in-law who works for ICE/CBP says that people always confuse the visa and I-94 thing. Here's the breakdown:

VISA: All a visa does is permits the traveler to APPEAR at a port of entry and request entrance to a country. It does not permit you to enter the country.

(This is why people without a valid required visa are denied boarding on aircrafts, cruise ships, etc.)

Document I-94 (white card): Is the document that officially permits you to enter the U.S. It states how long you can stay (in number of days) and the specific date on which you must depart the country. You may depart anytime before that date, but not after that date. The I-94 also specifies which activities you can/can not engage in while here (work, study, business, etc).

So, while a visa may state that you are eligible for entry for a period up to, let's say, 120 days, it is the officer at the POE who will decide if you will be admitted entry, and if so, for how long. The officer may grant you up to 120 days, or perhaps only 12 days. This is up to their discretion and they have the final determination. All the visa does is give the officer a guide to base their decision on.

Regardless.....look on the white card (I-94) to find her mandated departure date. Ensure that she leaves (means be completely off) U.S. soil by that date, if not a few days sooner. Even if she is one hour over the limit, and she may have issues coming back to the U.S. for up to 10 years. There are people who have difficulty reentering the U.S. because they missed their 11:30 red-eye flight out the to U.S. to go back home because their cab got into an accident or subway delays. DHS has no sympathy...they'll just say you should have left sooner.

Furthermore, and I'm not saying that you would do it, but please don't try to "flagpole"..meaning leaving the U.S. for a weekend "side trip" to Canada, Mexico, or the Bahamas, and then re-enter the U.S. with the intention to "reset" another entry period. Doesn't work like that anymore.

Although she was admitted entry into the U.S., -- any time that she spends in Mexico, Canada, or adjacent islands (Saint Pierre, Miquelon, The Dominican Republic, Haiti, Bermuda, The Bahamas, Barbados, Jamaica, The Windward and Leeward Islands, Trinidad, Martinique, Other British, French, and Netherlands territory or possessions in or bordering on the Caribbean Sea.) will count towards her admission period. So, basically travel to anywhere in continent of North America (save Cuba), will still count as a part of her stay in the U.S., and you don't want to get into that mess with a side trip to Cuba prior to reentry.

So trying to flag pole at the end of the stay will work, and can have terrible consequences.
(Example: Say someone is admitted for 90 days. If they leaves the U.S. and visits Canada on day 88 of a 90 day admission period, and is gone for 5 days, then they will have "overstayed" the 90 day maximum, by 3 days, and is now constitutes an "unlawful presence". The person will not only be disallowed to return to the U.S. in the future, but they may not be able to re-enter from Canada to effect an immediate departure from the U.S.)

Remember, the U.S. does not have formal exit procedures like most countries...you just hand the I-94 to an officer, and pray that it makes storage boxes, to be worked on by "Top Men". So, a dispute or question arises as to if someone remained in the U.S. past their I-94 authorization, the non-immigrants is at a disadvantage, because there is no proof (stamp, receipt, letter) on the part of the foreign national, that they actually departed the U.S. within the appointed time. (So, if someone loses your I-94, or if it is "lost" some other way, you may be SOL.) So smile at the person taking your white card...

Good Luck.