but my main issue is the war between the parents. I don't see why being able to hear (IF that is what the op does) would mean that the child would have to leave the deaf community, assuming that she can lipread, sign etc
A cochlear implant (CI) destroys any residual hearing in the ear where the implant goes. So when the outside bit is taken off the child is more deaf than without a hearing aid. Ditto for swimming, can't wear a CI in a pool.
Your child can not take part in contact sports afterwards, so can never play rugby, Judo, wrestling.
It is a major op sticking part of the device in to the cochlear and another bit under the skin, there is a risk of partial facial paralysis.
Facial expression is a big part of BSL. If effect it would be like a hearing person having an operation that might leave them talking like a dalek - unable to modulate their voice, give expression, surprise or even use some adjectives. What operation would you go through with those risks?
For me it would have to be life saving or fairly serious.
If a child is going to have a CI the best time is prelingually, the older the less benefit. 9 is quite late, adults have them fitted but it is the choice of the adult. In most families CIs, use of sign/oral education are discussed when the child is young.
CIs are not suitable for all types of deafness. The implant is in to the cochlear which is connected to the auditory nerve which sends the hearing 'signal' tot he brain. If that nerve is missing or damaged a CI won't make any difference.
That is not to say CIs are not a life changing invention. The posters who have children with CIs will tell you about that.
For some people it is the difference between existing in a hearing world and living in it.
I think there is a move in society to normalise disability to the point, sometimes, of not thinking about the individuals. This is thankfully changing, but, have a look at photos of children affected by thalidominde, in the 1970s they all had prosthetic limbs fitted, look at them now as adults in their 50s and few if any use prosthetic limbs.
This sounds to me like ex is just trying to hurt mum through dd. Why wasn't it discussed when she was 2? Why, if she is functioning well in deaf and hearing worlds why does he want her to have a CI?