This answer is intended to address the question and also include some general information to show the perspective that most people (including, I believe, Itai) have when answering these questions.
First of all, most questions about proof of funds are scams, especially if it's somebody you don't know that well asking to borrow money from you.
Nonetheless, I've seen plenty of questions with a legitimate funds concern, commonly involving entering the UK from Africa or South Asia.
It's important to remember that these proof of funds are (usually) fundamentally a question about income, not current bank balances.
Why is that the fundamental question?
Because people who can afford a round-trip plane ticket can probably afford to live in a place for X days, for any reasonable value of X.
Also, even if they can't and have to try and leave early or rely on some sort of local charity, the cost to the host country is capped by the duration of the stay.
If the host country still cares about this risk, then (like Itai says,) it's going to be proportional to the intended stay.
On the other hand, there are plenty of people who can afford a round-trip plane ticket and would like to enter a country to break the law by staying illegally to either work there or live with relatives.
These people are a bigger problem, since there is no cap on the potential cost to the host country (they die of old age eventually, but I wouldn't call that a cap.)
So why does your particular case say "scam" rather than "legitimate"?
Well, even if your friend really had to provide proof of funds, a sudden gift of X dollars wouldn't be likely to help, since it doesn't resolve the fundamental concern, which is that they may try and illegally stay.
This is why your request is probably a scam: if your friend actually had a proof of funds problem and they are using an agency, then the agency would know that a single large loan wouldn't help, so they wouldn't ask for it.
Some more-or-less legitimate questions about proof of funds
And some examples of probable scams
I hope this helps.