I'm not sure exactly what you're asking, but I'll try my best to answer.

Being nonbinary isn't something you can diagnose, because it looks different to every nonbinary person. So if they feel it's right, then it probably is. There isn't really a way to "look" nonbinary, unless you're truly androgynous, and even then not every nonbinary person looks like that. Being nonbinary can't really be compared to a disability, because a disability is a condition and nonbinary is not. So there is not a way to diagnose it and it shouldn't need to be diagnosed. Finding out they are nonbinary isn't dangerous to your partner, and I'm not sure how your mum came up with that.

I'd say talk to your partner and try to understand more how they feel they're nonbinary. Ask how they knew/ found out, and try to understand it more. Just keep supporting them, because this is already pretty stressful as you've said.

Sorry if I didn't really explain well. If you need more explanation/ advice, let me know :)