It sounds as if your father got sucked into this doctor's trouble vis-a-vis the ever-expanding federal conspiracy laws, the only way I know of for one person to be held criminally liable for another's actions. You use the specific term, "post-trial," so I am going to assume your father took his case to trial and, if you're asking about remedies, probably lost his appeal already as well.
If he has not appealed the case, stop reading here and go seek out the best possible appellate attorney you can find. In the federal system, it is very easy for attorneys from anywhere in the country to handle an appeal, so you have a large number of choices and should seek out the best one you can afford with whom you and your father are most comfortable entrusting his case.
Let's assume, though, that the appeal route has been tried and not worked out. If the appeal was denied less than a year ago or he entered a plea, your father has the opportunity, potentially, to seek relief in a 2255 action, which is akin to a petition for writ of habeas corpus. This is basically a way to allege errors that were not brought to the trial court's attention prior to adjudication of the case by alleging that the defendant's trial attorney was ineffective in representing him. 2255's are not easy, and if your father entered a plea, he may have partially surrendered his right to bring such an action as part of the deal to boot. However, for somebody who is down to his last best shot, it is a viable option.
Two important points: First is that these matters are very, very tricky procedurally and, although inmates often bring them on their own, if you have the means, you should definitely seek out the assistance of an experienced appellate attorney in bringing such an action. (By the way, it goes without saying that if he is still on direct appeal, he should find a good appellate attorney). The second point is that there are hard and fast deadlines/statutes of limitation involved, so do not delay finding an attorney to help with this because you can waive these actions.
I agree with Mr. Yurachek in that the 2255 appeal is the only remaining avenue for collateral appeal within the first year. There is the 18 USC 3582 appeal to modify a sentence similar to Rule 35 appeal. I would seek out the help of a seasoned criminal appellate lawyer. you can also visit the Federal Defender's site for more info on sentence mitigation and appellate application.
What, if anything, can be done depends on the procedural status of the case and on some other factual issues. There may or may not be a remedy to help him. A lawyer familiar with federal criminal matters would have to review the situation in considerable detail. There is no easy answer to your question.
The first thing he needs to do is consult with an attorney regarding the procedural aspects of the case. As already pointed out, where the case is dictates the options available to your dad. An attorney will be able to speak with you about what options are left and if they are worth pursuing. It is difficult to say what can be done with the facts you provided. I'm happy to discuss this with you further if you have any questions.