Solutions for dog's pica

I do drop-ins for 2 golden retrievers every day. One is still a pup ~5mo. The problem is that he is obsessed with eating mulch, dirt, mushrooms, certain grass, etc. to the point where he ends up getting sick and/or vomiting.

The owners have a lovely fenced-in backyard but some days we end up spending 3/4 of the visit inside bc I CAN.NOT. keep this dog away from this stuff. I play with them and try to keep his interest but he inevitably runs off to one of these spots and then just bounces from one to the other until I have to tempt him inside with a treat.

I've put him on the leash and we spend the time walking around the yard but he cries the whole time bc he wants to play with their other (older) dog. The owner has also indicated that they'd like him to spend time running/wxploring freely since he is still a puppy.

I've talked with the owner about maybe doing chili powder or hot sauce on some of these areas tos see if that'll deter him -- have any of you had a similar issue? Any suggestions?