My 20 year old is wearing diapers to control his bed-wetting but its not working. Any advice?

Get a bedwetting alarm to help him.
They carry alarms for adult bed wetters.
Good luck!

Can you tell us a bit more about him - what the cause of the bed wetting might be? Is there a physical or mental disability?

I only ask as this is a subject I'm very experienced in as my 24 y/o son has sevear Autism and functions at a 2 y/o level - wears disposable underwear at all times.

However - what works for me may not work for you - depending on what might be causing the problem.

We have used two solutions. Best (and most expensive) is Abena brand, Abri-Flex Premium M3. they fit my 170 lb, 6 foot husband with 36" waist. They advertise that they hold 84 oz of liquid. My second choice is to add a flow-through type "booster pad" inside the disposable brief that you currently use.
Good luck,
Jamie

Could you tell us a little more about your friends condition? Has he seen a urologist? Is he handicapped in some way?
You've probably tried these suggestions if this has been going on awhile.
Restrict fluids after a certain hour, use pads inside pull ups, put a plastic sheet on the bed to protect the mattress, set an alarm clock to wake him during the night to go to the bathroom. Let us know what you've tried and what other health issues he might have.
I know this must be difficult for him and you.

please help