But if he has the photo, he no longer looks like a crazy drug addict regardless. He looks like somebody who was irresponsible for jumping to conclusions but ultimately reasonable. I think their only hope is in getting that photo back.

I forgot it was a mediator. But still, you could follow him around and find a good moment. Maybe he's sitting at a coffee shop, or at a restaurant. Ideally somewhere with a reliable security camera so the full footage can be found later. You sit down next to him, say "hey I'm going to the bathroom can you watch this bag," place it beside him, take the photos during that exchange, then say "thank you" and leave on the way out. Then you'd just need to prove you never gave him the bag but just had him watch it for a minute. It doesn't solve my first issue, of course.

Good point on an outside PI. It's still hard to imagine Howard choosing based on, what, a flyer on his windshield or something? Rather than a referral of some kind from someone he knows? He's in divorce proceedings, guessing the family law person he hired has PI connections.