Dato
In my opinion I don't think he should teach her nen. Toga's quirk is already pretty powerful in the manga. I think he should instead train her on how to fight and or use her quirk better. He could also teach her assassination skills because I think that would fit her character better. I just feel that when you start teaching other people nen it would in a way weaken Killua since the one thing that he has on the people of that world is suddenly not unique to him.
I just feel that if she trains in using nen then she probably won't reach the full potential of her quirk. I understand that she most likely won't be a genius at using it and would have to train pretty hard but again that is a waste of time when she could be developing her quirk further. In the manga she got to the point where she could copy someone's appearance and quirk but she only got to that point by working with the league of villains and if she isn't going to join them in this story then I don't know how she will develop her quirk to that point without focusing on it and training on using it only. You have to remember in the hxh world people aren't born with special abilities for the most part and they live in a dangerous world. The hunters are forced to learn nen if they want to survive their jobs and fights. In the mha world the potential uses of their quirks could be limitless as long as you train enough and taking time away from that to focus on something that she doesn't need would be detrimental to her potential. Anyways I'm not gonna be mad or drop the novel if he does teach her nen I just feel like he can instead teach her more useful things that compliment her quirk instead of nen which would probably take a while.
Spelloyal:killua is a genius in a million even in the world of hxh where there are some talented bloodlines of nen users. Just as he is special in HxH he would remain the same if everyone in BnH learned nen, because nen has much more growth potential than quirks. As long as Himiko's talent is not as good as that of killua I think everything is fine. The author can make her learn the basics quickly due to the similarity of quirks with nen, but give a slower growth rate than killua. Even if she has a talent equal to that of killua, he already has the advantage of having started training long before her. Just turn her quirk in a hatsu and dont let her create a new hatsu.
Teach anyone nen And your mc is simply not interesting anymore. What makes him special and unique is not being from another world, but having a type of power no one else in this current world has. Take that away and it’s dull (in my opinion). Just teach her combat and assasination techniques. Skills on escaping and fleeing. Toga isnt a fighting type to be able to contend with the more stronger people. If shes very good at fleeing, then thats fine. Personally , Giving her nen will make me lose interest on the story.
Do not teach him "nen" please, it would take away the advantage of the protagonist, also you will probably take away what makes the protagonist special, toga can defend itself, it does not need to use nen. She doesn't need to be able to beat everyone she can escape with is enough, plus she can probably take almost everyone by surprise most of them would fall after a stab