My plumber, who is an in-house guy working for my GC and has already done a couple questionable things, joined this new pvc drain to a cast iron drain line with a Fernco elbow. I believe it’s a 2” cast iron drain which ties into a 4” just on the other side of that joist. It’s already sagging which is an obvious problem. But would this be acceptable if it were just strapped to the joist to keep it in alignment? Or is using this type of coupling in this application just generally a problem? Is there a better alternative?

For context: The area is now particularly hard to access because the hvac supply and return trunks are immediately under it. This is just below the first floor, in a basement that we eventually plan to finish (and this would be captured in a bulkhead framed around the hvac trunks).