Can a friend borrow my U.S. Debit Card with my consent?

This is completely legal and people (against good sense) do it all the time.

The three possible issues are:

1. (Somewhat likely) The "friend" takes your money, now or later, or otherwise leads to a security problem for you.

2. (Unlikely) He's using it in a grocery store or something and acting real twitchy and doesn't have ID matching the card (some stores check ID to match debit cards), and they call the cops and the cop wants to make sure he didn't steal the card. Can be cleared up, but nobody likes police hassle. It's perfectly legal, however. And they're not going to "report it to the bank," no one gives a crap or has time for that.

3. (Unlikely) Sure, this is probably against your bank's T&Cs. The only way they would know, though, is if their auto fraud detection kicks in and they freeze the card because the activity in a different country from usual looks suspicious. Just call ahead and tell the bank "I'll be travelling to the US" so they turn that off. In general that would just lead to a temporary freeze you could unblock, but if in the very unusual case it lead to more inquiry and they found out you had loaned your card, they might (probably would not, but might) cancel your card and/or account, but they're certainly not allowed legally to keep your money. While you could add them as an authorized user, then you're just doubling down on the much more likely problem #1, so I'd recommend the don't ask/don't tell method. People do this over here with family members &c all the time.

TL;DR this is fine, tell them not to use it in stores where they would be asked for ID and call the bank and say you'll be travelling to avoid any complications.

If the reason for doing this is to avoid (or reduce) bank fees when your friend uses their non-US bank card in the US, then they are better of getting a pre-paid travel credit card.

These (generally) work by loading money in the user's home currency, and then transferring the money into US Dollars (in this case), or other currencies. You can then treat it like a regular debit/credit card. Others do not require you to swap money, which saves the (minor) expense of swapping any unspent money back to your home currency.

There are several different providers, and they have different fees. Which one is best depends on how often you travel abroad; Fees can be annual, loading, swapping or usage. Annual fees generally benefit those who travel aboard extensively.

It is highly unlikely that the bank will act unless some sort of crime is committed. That issue is almost unheard of, unless thousands of dollars are involved (ie, hotel fees might trigger some issues.) Many cards will require you to 'unlock' transactions which are from unexpected locations (eg a different country). The real question is if your friend has the ability to pay you back, why would he/she need your money in the first place? The advice to never loan friends and family money is wisdom. If you can't afford to lose everything in your account, don't do it. The other issue is theft. If you give your card to a friend and it is lost or stolen, you may never recover any money. This could affect your credit report as well as impact job interviews (many companies do background and credit checks). Since your friend can buy a debit card pre-loaded with money, it doesn't make much sense to me why your friend needs you to give him/her yours.