When I was running both at the same time about five years ago, yeah, I personally found Jenkins to be more operationally expensive than a small postgresql instance. It required more memory, more CPU, and more time to set up and keep running as desired. Maybe things have improved or maybe it's just a YMMV thing, but yeah, I would have said the same thing as your parent without thinking twice.

Edit to add: oh yeah, I just read another comment that reminded me that it also ate up lots of disk space as well.