I think you have two problems here.
One is the fact that you know this is likely to lead to some kind of scam or deceit that the parents need to be protected from, and how best to do that, particularly if they don't accept they need protection.
But before you can really address that imo the more pressing problem is that you're very reluctant to accept it yourself. You're obviously an intelligent woman, and the consensus on this thread is unusually clear, together with some very knowledgeable expert opinion.
I think this individual is someone important to you despite everything that's happened in the past, and understandably it's hard to believe they would stoop so low. Presumably this is also happening hot on the heels of believing they were dying imminently and now having to reconsider that bombshell too.
I think you need to take some time to let this excess of news - good, bad and ugly - just subside for a few days, let it sink in, and fgs be a bit kinder to yourself about your reaction to it all. You seem to be judging yourself for your wicked thoughts, on the basis that suspecting him of lying would be a terrible thing to do if actually he's telling the truth - except that he's not telling the truth, and given the painfully obvious red flags, you're not a terrible person to be thinking along these lines. On the contrary, your love and concern for his (your?) parents shines through everything you've written, and says much more about the kind of person you are.
Once the facts of the situation have had time to percolate, I think you'll feel a good deal clearer about what to do next, whether that means tough love with the parents, or some more indirect way of protecting them and/or investigating him. Right now, I think you're in shock.
One other thing that occurred to me is to wonder if you're not the only one with doubts. I find it curious that the father would go to the trouble of writing the emails out in their entirety if he accepts them at face value. Is it possible he too is wrestling with the possibility that there's something not quite right about what you're being told? When you've all taken some time to reflect, I wonder if you may find you're pushing on a partly open door after all.