If we temporarily disregard the restriction that Herbert does not sit at the end, there are 8! arrangements of the passengers. Of these, 7! have Herbert sitting at the left end, and 7! have him sitting at the right end. So there are possible arrangements, which we will assume are all equally likely.

How many of these arrangements have Herbert between his pals? Duct tape Herbert and his pals into a single unit with Herbert in the middle. This can be done in 2 ways depending on which pal sits on the left. There are then 6! ways to arrange the group of pals plus 5 aunts.

So the probability is .