I remember there was a movie "splitter"? on the similar subject

There are two things:
1. Without written consent, one should not tape another person. there may be laws governing this. You can google or shout for help from single-old-cat or consult a lawyer.

2. As to you find out this from his private stuff, this "probably" ok, as the constitution grant the right of private investigation to everyone. Again, I am not sure, You can google or shout for help from single-old-cat or consult a lawyer.

After you found out the anwer to all these, you probably can take actions. Anyway, make sure that he did not make extra copies.