Use of a drill bit to create an opening in the floor joist would be one solution. Moving the plumbing fixture a few inches may be another solution. Having the architect alter the floor plan or the framing plan would have avoided this problem in the first place. Unfortunately none of these alternates occurred, now the home inspector and home buyer have a problem.
Is corrective action necessary? In a word yes. Perhaps one will be lucky and during the time one owns the home no movement of the structure will occur that requires corrective actions. Even though that may or may not occur one of the real problems is when the home is sold next the same problem will come up. Then today’s home buyer will be the one to make excuses to the next buyer that the next buyer may not accept.
The time to cure the problem is now. Water supply pipes, waste water pipes and wires will have to be temporarily relocated in order to allow the plumber to relocate the drain pipes in order to allow the carpenter access to replace the joist or install another like joist next to it.
Prior to performing your own plumbing work check to determine how it will impact the structure of the home. If you save $900 by installing your plumbing however if will cost $2,500 to repair the structure there was certainly no savings at all.