It is not illegal to take his picture or His Mistress . I would be subtle (unnoticed) if possible. Take pictures of where he & she works, vehicles (any asset, boat, car, jet ski, home, ect) & licence plate number (of both of them), to include mistress. This will help obtain his Mistress name. Any public picture is fair game. Resturants , clubs,functions, parties, parking lot, driving together, their open curtains with a view of them both. Video of them hugging, kissing, ect. Make sure your pictures are clear and recognizeable, not blurry, far distance. Does he own his own business? Get a business card. Or web site address. Lastly…Take your time.
Note, this is not a criminal forum, and I am do not practice criminal law, but it is my understanding that taking pictures of the couple in public to assist with claims and allegations such as alienation of affection and proving adultery is legal as this is what private investigators do regularly.