How do I tell my friend that she needs to wear protective undergarmets or depends at night?

First of all, let me mention if she is regular try to take her to the bathroom periodically to help from having "accidents". I had a hard time getting my mom converted to depends. At first she argued. But I gradually weened them onto her. I didn't use the word depend/diapers.. etc. I called them underwear. I took them out of packaging and put them in her underwear drawer. I would put them(depends)on her first with her actual underwear over top. During this time I also bought her a bedside commode and put by her bed for at night. A month or so (being that mom is forgetful) she got used to them and I solely used them and no other underwear.
I understand what you mean about finding messes and changing bedsheets. My mom recently started to get "smart" about the depends and she would take them off or pull them down and go anywhere!!!!! So, I bought the diapers with the sticky tabs on the side for her to wear at nite. I use a little masking tape for extra security as well as a depend overtop.I have also made her PJs that are one piece and fasten in the back(was a great suggestion from a member on here).By the time she fiddles with her PJs and her diaper she already went in them. Saving her bed linens from the mess. So, now I can get some peace and not cleanup pee and poop all day and nite. She still has her good days and bad. But there is more good days now!!
Also if she is going more than normal, check with doctor to see if she has a Urinary tract infection!
I know, trust me that this is so difficult to deal with. My Moms "accidents" were destroying the house.
GOOD LUCK!!! I hope my suggestions might help you!!