assuming that sensei has decided not to take specific action on that student--does that mean that tori should move out of the boundaries of the specific technique being taught, and start implementing his/her repertoire of henka waza, kaeshi waza or atemi waza on that person? or is that just being spiteful and disrespectful?
That would be the worst thing to do. It will just turn into a pissing contest that is categorically NOT aikido training. Too much competition and ego involved.
When I run into this kind of person, I just adopt an attitude of "I guess you're just too good for me!". It's a loss of opportunity for both people, but I'd rather waste my time than end up injured. Hopefully you get to switch partners and UFH can go bother somebody else for a while. I really don't care what somebody like that thinks of me or my ability.
If the person is in my circle consistently (i.e. wasting my time a lot), then it's time to find the most aikido way to present an attitude of "I'm not training with you anymore because you're an idiot".
If I am teaching I will teach them how to take ukemi. If I'm not teaching AND I'm significantly senior to the person AND I quite like them, then I might find a kind way to enlighten them. In all other cases, they are free to bask in their own ignorance and stagnate forever.