I was curious as to the thoughts of others when it comes to the role of a senior student assisting with instruction. In your opinion what are his duties, (provided the head instructor has given that person permission to teach of course)
For me, it would usually depend on the rank of the person helping. Usually once you're around the green belt level, you start helping with classes. There were times when I'd have the helper do the warmups, stretching, punches and kicks. If we broke up into groups, they'd take a group and review requirement material with them. Sometimes I'd just have them review, and I'd move from group to group, doing the actual teaching of new material. Other times, I'd give the helper something to teach, and then make my way around the room, making corrections, helping with questions, etc.
and how much time should be spent between progressing in one's own training as opposed to simply being there to coach?
IMO, there needs to be an equal balance. I say this, because there was a time, when I would show up, with the intention of taking a class, and end up having to help. So, in the end, during that week, I'd average much more teaching time, and less of my own training time. This isn't right and will lead to burnout.
As I said, you need to balance things out. If you show up for a class, intending to be a 'student' rather than a teacher, you shouldn't have to help teach. When would you get your training time then?