In my humble and non-medical opinion, I think the changes in your friends mouth warrant a visit to a dentist or doctor immediately.
You say you have begged him to see a doctor and I gather he has refused. Do you think it might work if you made a dental appointment for him and said if he won't go for himself, to please go for you?
Perhaps if you did some research on the damage dipping can do after that many years and printed it out for him to read, that might get him concerned enough to finally relent.
Being a grown man, you are stuck with leading the horse to water.........
For his sake as well as yours, I hope you can persuade him to go.
I wish you the best
Peace
Greenlydia