You need to force yourself to replay the images as frequently as possible, until you become desensitized to them. It is called a paradoxical approach and it is usually the one that works the best. Since the video of his having sex with the wife of his older friend is available, you can watch it until you become desensitized. Paradoxical approaches usually win, because if you try to force the images out of your mind, you are likely to see them come back only more frequently and more intrusively.

He did lie to you, apparently, since you asked him about the number of partners looking for details, right? Because if you were not looking for details, but only looking for the figure, then he did not lie to you, technically speaking. You know better how the conversations unfolded between the two of you in the beginning of dating.

Also note that he might have been embarrassed since his sexual rite of passage happened under non-standard circumstances, and he might have been afraid of negative judgment coming from you. If you want to hear true disclosures, you need to make sure that you position yourself as a safe, non-judgmental person.