My father-in-law has had SEVERAL incidents of forgetting where he is, driving to one place and ending up in a completely different place, taking eight hours to drive five miles... Part of the problem is my mother in law has her head in the sand. She had him an appointment to be evaluated, and was not going to tell him what it was for (he was in a major wreck a year ago and has been at the Dr. almost daily, so she could have pulled it off), anyway, she ended up telling him what it was for, and of course, he refused to go. He's still driving and my concern is that aside from getting lost, he's going to hurt himself or someone else. Is there anything a non-spouse can do?
So, these are your inlaws? Where is your spouse this discussion ?
It's true that you can't physically force someone to be evaluated, so getting third party official support is the way to go. If you get support from the DMV by reporting the driving issues maybe they will revoke his license until he's been evaluated.
The original plan was a good one, so it's too bad that his wife told him why he was going to the doctor. But you can't help what's been done. I'd try calling the DMV and reporting him. Other than that, you may not be able to do too much since you aren't his spouse. If his wife will allow you to literally take away the keys and/or disable the car that may help but he'll make her life very difficult so she's likely to cave.
Good luck with this frustrating problem. He needs to be off the road and you need official backup to do it. It's easy to watch videos that tell you to "talk with him," but as you're finding out, it's not that simple. I hope your DMV will back you up.
Keep us posted on how this goes!
Carol