@doctormike might have some insight (definately in his wheelhouse) and maybe a referral. Certainly worth getting it checked out!
Hard to be precise without knowing what the issue was and what the operation involved. This information might be reconstructed if the OP's husband had an ear specialist (an otologist) examine his ear and check his hearing, you don't necessarily need the original records if they are unavailable.
Yes, someone might refer to ear tubes as a "prosthesis", but I think that's not necessarily what was done. Ossicular prostheses are artificial ear bones which are used to reconstruct the middle ear when there is a problem due to congenital anomalies, cholesteatoma, trauma or other conditions which affect the ossicles.
In general, if someone has an intact eardrum and can equalize in relatively shallow water, it's OK to do dive training with the understanding that (1) equalization may be more difficult in deeper water, and (2) if that happens, the diver must absolutely commit to NOT pushing through this but going slow, stopping, and even ascending a bit to clear. And of course thumbing the dive if they can't equalize.
The one issue that might be relevant here (and why I would recommend a specialist evaluation), is that some prostheses involve the juncture between the middle and inner ears. Stapedectomy for otosclerosis is one example, where the third bone in the chain doesn't vibrate well, so it is replaced with an artificial one. But this may involve a potential leak of inner ear fluid. If you blow out your eardrum from dive related middle ear barotrauma, that usually will heal or can be fixed. But if you get an inner ear injury from diving associated an abnormal communication to the inner ear (i.e. perilymph fistula or after some types of middle ear surgery), you can end up with permanent deafness, as well as severe vertigo at depth.
Now otosclerosis generally presents in early adulthood, but there is something called a congenitally fixed stapes which can also be treated surgically, and may be done in childhood.
So as with most things, the devil is in the details. See an otologist (better than a general ENT doc for this), if possible. If you want to PM me with your location, I may be able to give you a name.
Dive safe!
