He is never going to be able to "prove it" as tech is at a state where anything you present as legit is going to questioned and doubted.
In the old days you did not have the tech to actually capture proof. Now tech is at a state where you can fake anything, and you can't trust what you see and hear.
I had an idea once to make a pinball machine with EVP's, but I can't make a pinball machine and if I could the quality of anything I captured would not be sufficient quality. If I had to go to the trouble of making it presentable that would pretty much be faking it anyway.