My friend has for 1 ear a hearring aid and also had as a child a lot of problems with his ears. Before I did a trydive with him, he went to a doctor to ask about his ears as he told me he was not able to clear his ears, but he also did not had problems in a plane or so. The doctor did not find anything with eardrum or not being able to clear his ears, so we have tried to dive.
If you cannot clear your ears, you will feel it at 2m depth, and then normally nothing has been damaged. So we just started to try and went on the trydive to 3m. Not deeper, but this had to do with his problem that he wanted to get to surface every time. He was no natural in diving.
Now he has done around 70 dives, I have taken him to 42m depth, he has done several solodives already, so yes, he can dive. But 1 ear is still sometimes a problem. He is very sensitive for running ears. And then he has to stay out of the water for 1 week or so till it is over again. He went back to the doctor, but they did not find anything. Sometimes he cannot clear his ears, and always we have to go down very slow. If he cannot clear ears, then he goes out and on holiday he tries it the next dive again.
We use eardrops with tea tree oil before the dive (are sold for divers), rins with clear water after a dive (not too cold water) and at home a mix of alcohol, and vinegar. And of course always just in case he has some drops with antibiotics if there is a real problem. But with the easy over the counter eardrops most times all goes fine. Last year in Indonesia we did 14 days of diving, he 3 dives a day (I 4 or 5), and his ears did hold it. 1 day he thought he had some wet things coming out of his ear, so skipped the first dive, but tried the second again and then in the evening the ear was ok again with the over the counter eardrops. So the antiobitcs were not needed.
So if we go diving, we need to have a divecenter that does not make a problem if someone cannot clear ears and go out again. Sadly there are not a lot of divecenters that don't destroy other divers dives then. My friend can go out on his own, there is no guide needed.
Also we need to have a divecenter that has no problems with 1 diver getting down very slow on boatdives. (rest can go, he will follow later).
To find these divecenters is the worst thing about 'bad ears' for diving. A lot are hurrying too much. Diving on your own don't give these problems. We dive for 99% of our dives without use of a divecenter, but I know a lot of divers only dive with guides.
So if your friend is cleared by a doctor to go diving, do it very very slow. Never accept rushing by an instructor or guide. And rinse ears after diving. For me clean fresh water is enough, but if I don't do that, I also get ear infections in some waters if I don't rinse after a dive (here in summer, a lot of people get the otitis externa when water is around 20 degrees, or in some caves) and I have ears that can clear almost always even if I have a cold, cough and a fever together, so normally don't have problems with ears.
And for people with ears that don't clear easy, negative entries are not a good idea of course.