XR250, "once the work is done"? So have these piles already been installed? If so, it sounds like the homeowner maybe has buyer's remorse, and there is probably not much you can do for them. On the other hand, if the work has not yet been done, then I think you are in a good position to perform a structural condition assessment for the owner and offer your professional opinion with regard to any foundation issues, and whether and to what extent pile underpinning will be beneficial as a remediation/repair procedure.
To your original question about whether it is unethical for a third party engineer to design foundation repairs based on information about existing conditions provided by a foundation contractor that is paying him/her for the design. I personally think that it is, but, unfortunately, the profession in general doesn't seem to think so, since I think that the exact scenario that you describe occurs often with little thought or pushback.
I recall reading an ethics case brief from one of the professional organizations (e.g. ASCE or the like) several years ago that addressed a similar issue. In that case I believe the issue had to do with an engineer relying on site photos and information provided by a contractor to address an issue in the field during construction of a building. If memory serves, the panel's opinion was that the engineer should not have relied upon the contractor's photos and information, because the engineer did not have responsible charge over the contractor. I wish I had a copy of this particular case brief, but I have googled for it several times over the years and have not been able to find it again. Does the case sound familiar to anyone else.