It’s pretty simple, you need their account number

Then you purchase a money order and mail it to their debt collector with instructions it is to pay off the debt. Getting the account number is the hard part.
If you know who they owe, but not the account number you can also go this route. You get the money order, make it out to the company for the amount you want to pay for them, then you mail it to the individual with no return address with a type written note that says “You have been given a random act of kindness, pay it forward when you can” or something similar. Since the money order is made out to the company they will have to pay it to the company and they can follow through making sure it get credited right.