I love the answers that surprise me. I had never doubted the effectiveness of orthotics. (To be fair, I've never needed to care.) I thought it was part of evidence-based medicine. I went to find the evidence to show this. However, the literature seems more divided than I expected.
When there is controversy about the effectiveness of a treatment, I look to the Cochrane Collaboration, who produce high-quality meta-analyses of smaller studies to draw more robust conclusions.
There was a recent such analysis done that sounds (without knowing the details of your father's medical history, nor having the skill to understand it if I did) perfect for this question:
They looked at two available studies, and warned of the biases they found.
There were no important differences in knee pain and function in people given foot orthoses as well as physiotherapy when compared with people given physiotherapy only. Results for knee pain and function did not show important differences between foot orthoses versus physiotherapy.
On the basis of the available evidence we do not recommend foot orthoses for adults with pain around the knee cap.
There may be some areas where foot orthotics might have a benefit.
This review concludes that they are largely ineffective at treating pain or preventing injury, but that there is some evidence of helping with plantar fasciitis, posterior tibial stress fractures, and patellofemoral pain syndrome. Mainly, it suggests more high-quality evidence is required.
There are also unrelated situations where orthotics may assist.
The New York Times had an article discussing the controversy.