Why would an uninvited guest be asked to teach or even to help teach and how is not doing so disrespectful?
The way I was trained, if someone very senior like that came as a guest, he would always
be asked to teach. (Often, but not always he might decline.) Even more so when the instructor is so junior.
On the other hand, since the instructor was "subbing" for his own teacher, it gets a bit sticky. If he didn't personally know this guest teacher, he might have felt he was exceeding his brief to invite him to teach. There are a number of senior Aikido teachers I wouldn't let within a mile of my students. What if this guest had injured one of the students?
If one of my instructors had a fellow he didn't know at all from direct experience take over class for him, he had better be following the recommendation of someone he trusts implicitly. When I ask someone to teach, he or she is responsible in my place. I would be unhappy of something happened because my student has passed that responsibility to a stranger.
On the other hand, if one of the senior folks from our organization or one of my personal friends dropped in when I was away (unlikely as they would normally let me know first) it would be fine with me, and really expected, that my designated instructor offer them the chance to teach.
In the situation described, as Wayne is an old timer and known to the dojo folks, I would have expected the Nidan teaching class to have offered to have the guest teach. The ultimate responsibility for the guest's behavior would then reside with Wayne, who provided the introduction, if the guest behaved badly.