How much money do you need to have on hand when entering USA?

Enough to support yourself for that period of time you're in the country.

It's frustrating, but there's no fixed number. They'll likely ask your friend about her trip, where she's going, what she's booked and how she'll support herself. It's just to make sure that she's thought through how much she'll need to spend and support herself, without getting into financial difficulties.

I've been in to the US half a dozen times, and while I've been questioned, I've never actually been asked to show proof of funds. And certainly you don't need much cash on hand, if you have access to more through your credit card / bank / traveller cheques.

Short-version: there's no exact figure, just evidence that she'll be able to cover herself for her travels.

It may sound strange, as many travelers here point out that they never been asked if they brought enough money while entering the US. But the U.S. Customs and Border Protection states that:

Travelers visiting the U.S. must be able to prove to a CBP Officer that they have sufficient funds (i.e. credit cards, cash, travelers checks, money orders etc.)

Good thing is that credit cards are accepted as a mean of sufficient funds. To be able to give some proof, everytime I travel to a foreign country I bring with me a print of my last credit card bill where it shows my credit limit. Never had to show it to any immigration officer, but is a simple measure and should work as a proof of funds.

I traveled to the USA this December from South Africa and they did not check how much cash I had. I actually had 0 dollars on me. All I had was my credit and cheque cards that I could use across America. There was no issue.

So unless you find some text on the visa website you do not need to have hard cash with you at the time of entrance.

Your friend will need "access to sufficient funds to support themselves during their stay". This can be as low as "nothing" if a local resident is waiting outside to pick them up, "access" in this example being your wallet. A lot will depend on what the immigration agent thinks of your friend (and/or you) when they approach the desk. Clothing, accessories, baggage are all part of the process. If you look moderately well off (not rich, just comfortable - the trip might be expensive for you but it's not beyond your means) then they won't ask further. Possession of a credit card will be assumed unless you provide reason to assume otherwise.

If you look like a "USA on $20/day" backpacker, a refugee, or anyone else who may need to do things like get an (illegal) job to pay their way then they will want to see some cash and/or a functional bank card, itinerary, local contacts, return ticket etc.

So just tell Miss Chile to dress nice, check her hair before landing, smile and be pleasant. You do the same - be the person immigration will take one look at and say "no problem". An Armani suit might be a bit much if its a weekend arrival but Hugo Boss casual wear will work. LL Bean at least.

I've travelled to the US from Ireland 10+ times on a visa waiver programme (ESTA) and never asked about my funds or cash at immigration. I almost always travel with no USD$ (sometimes I have about $50 for emergency's, sometimes $0), just my credit and debit cards.

Never been a problem. Never been asked ANYTHING about funds. But, I am a 40y/o white guy from a western country, so your mileage may vary.

Interesting. I am from Europe and every time I enter the US on ESTA the immigration officers ask me how much money I have with me. First, I made a mistake and told them the truth that I have $200 and they didn't like it. They asked me after that again, so I figured it was an issue, so I had to tell them I have $200 cash and about $1000 on my European atm card (which wasn't really true). Luckily, they didn't want a proof. They let me go, but asked me everything about what job I have, etc. before. Last time, I was coming to the US from Bahamas and they asked me again how much money I had. So, they do ask...