(I'm not that person, but I have opinions on this. I'm also not deaf, but I am autistic, and the autistic community has a lot of parallels to this that I can't help but notice.)
> because she can't sign as fluently as a child who grew up signing, and the majority of the community is against implants
This is two separate issues.
I view the first as like any other language fluency thing: should English-speaking communities ostracize members who are less fluent in English? If they're extremely non-fluent, to the point of not being able to communicate at all, then there is no morality here; it's kind of just an unfortunate thing that happens as a result of non-communication, like if I (a non-Mandarin speaker) showed up to a Mandarin-speaking social group and tried to be their friend. If they just have a thick accent and word things oddly sometimes, doing so might be an asshole move.
The second is entirely the community's problem. People should, at least, have the option of curing (even partially) their own deafness, without facing weird stigma from other people who choose not to do so. It is ridiculous to ostracize other people for doing things that they feel improve their own lives, even if you would not make that same change to yourself.
In our universe where we don't yet have a good way of doing so, what this means is that research into developing better implants is a good thing. Forcing implants on all deaf people would be immoral, as would forcing any other medical thing on people. But just making the option available is not.
I don't know how this translates to children too young to meaningfully consent, and I will stubbornly refuse to form an opinion on that because it depends on factors that I know nothing about. I don't think it matters, because it is immoral to hold back things from consenting adults solely because we haven't decided whether children should have them too.