Would your friend be open to just going to an ENT doctor for a physical check of his/her ears for a possible wax buildup. Could be impacted wax that is causing the problem and not an actual hearing loss. No need to get tested at that time…see what the doctor says. Happened to an aunt of mine years ago. She went out and got a hearing aid and I still couldn’t hear . Someone told her to go to an ENT doctor and lo and behold, she had an earful of impacted wax. Her hearing problem was solved.
If it is an actual loss your friend will not succumb to your insistence . I would not shout anymore. Speak in a normal voice and shift the frustration from you to him/her. How does your friend do with movies or tv? Tell other people not to shout. Your friend will eventually (hopefully) come to their own awareness when they realize they are missing a lot. I think shouting and constant repeating is a form of enabling. Won’t be easy for you and might cause friction between you.