I know when a new battery is needed so I'm looking for other answers. My FIREX 12000 series CO/Fire detector beeps only a month after replacing the battery(beeps once every 60 seconds). Sometimes it seems to take a break from beeping. It is 1 in a connected series of 7 in the entire house - AC with battery backup. It is the one closest to the kitchen.
Put a plastic liner on the mattress, underneath the sheets.
I would go to walgreens or walmart and purchase a bed pad and they are big in size and put it under the sheet in the bed he will be sleeping!
Tell him straight out to wear the diapers or find a hotel. Tell them it's your house and you can't afford to replace another bed. It's harsh, I know, but it sounds like a problem that been around for a while and you have to try the firm but fair method.
Go get some diapers and have them on hand. Give him one chance and if he wets the bed then he needs to wear diapers the rest of the time he is there. One wettting will not ruin the mattress. You will have to clean it, then insist that he wear diapers to bed.
I have been through many OSHA inspections in my life. Compliance Directives and Compliance Guides on the above page and read the frequently asked questions too. You will have to read what is applicable to your business or do a search on the OSHA site for your industry.
