If the $400 check is a one time-thing, they’re better to keep most of that money and make the minimum credit card (“CC”) payment on time. Rather than feed the whole $400 into the CC. That’s assuming they’re disciplined and won’t blow the saved-out cash on additional non-mandatory spending.
The key thing is that as bad as paying $6.50 in interest each month is, paying that plus paying a $30 late fee plus having the interest rate doubled is worse. Which is exactly what *will * (not might; will) happen if 30 days from now they have a fresh $400 CC balance from new charges and no fresh $400 check to pay it.
As somebody mentioned, another issue is the CC’s credit limit. You don’t want to blow through that, or have accumulating interest push you over that. This may or may not be an immediate issue, but it pays to check. If you are paying at least the minimum then the balance will be going down expect for additional purchases. And they’re unlikely to approve a purchase that’ll put you over the limit. But if they do you’re the one who’s screwed, not them.
Bottom line, as others have said. A typical high interest CC is a very crappy source of rainy day funds. For people who are already having trouble earning enough money to meet their fixed expenses it’s easy to use a CC to dig a hole in two months that takes 3 *years *to dig out of. And that’s assuming nothing else goes wrong in those three years. Which is pretty darn unlikely.