Well… it is quite possible in the current academic context that some instructors need to stitch together multiple jobs.
Is this person tenured or tenure-track? It would be unusual for this person to be tenure-track as such a workload would affect other conditions required for tenure (v.g committee work, supervision, research etc). If this person is tenured it will be harder for the university to do much unless the instructor is in breach of contract, i.e. obviously does a poor job or is explicitly forbidden to hold a position outside of their current institution.
Keep in mind it is entirely possible for one to have a full-time limited-time contract position at one place without this restricting work outside of the contract. In particular, it could well be that this person does not want to abandon courses taught elsewhere so that, once the contract runs out, they have multiple cushions for a soft landing.
Ultimately the only thing you can do is provide constructive but critical comments at the course evaluation. While these are not necessarily available to the Dean, they at least point to the issue.