It would be valuable to know if your friend has current health insurance, though any bills for which he would be responsible will follow a similar pattern.
It can vary from facility to facility, but in general an inpatient hospital will offer payment plans (particularly in the case of bills that are large and for services already rendered; your experience would likely be different if trying to schedule a surgery in the future, for example).
In all likelihood your friend's medical bills won't even be finalized at the time he is discharged. Medical coders and billers will review clinical documentation to determine which specific services were provided, and for what purpose, and then the cost of those services is set by:
(1) your friend's insurer's negotiated rate for that procedure code, or (if the provider is entirely out-of-network, the service is not covered by the insurance contract, or your friend is not currently carrying insurance),
(2) the hospital's charge master, or (if they allow such arrangements, which many do across a variety of circumstances),
(3) a reduced rate off of the charge master.
Any reductions in the total charges or minimum monthly payments are likely to be governed by strict policies internal to the hospital. So, for example, while the hospital will always take his money there may be a minimum monthly payment which, if it is not met, will shift the account into poor standing.
Your friend can expect a bill in the mail any time after his discharge, and when the first bill arrives it is important to contact the billing office to make whatever payment arrangements are appropriate. Additionally, it's possible (even likely, depending on what he is in the hospital for) that other medical providers external to the hospital will bill separately. These external providers will have their own policies for paying down a balance, but as with the hospital it is important that your friend contact them to receive any accommodations.