What is he chewing on the trees......the bark? Or are these small trees that he mistakes for twigs?

She should redirect him with a large, fresh bone from the butcher and supervise him more when he's outdoors.

Other option, wrap some wire mesh around each tree until he outgrows the chewing stage, or is adopted.

Spraying the trees won't accomplish much.

Yes, chewing the bark . Her trees are at least 30-40 YO so doubt the puppy has exposed the Cambium layer

fence off the trees!

I have the same problem except my dog tears off the bark of my palms. Anything made of wood she will find and chew on. So annoying but she never destroyed anything in the house so that’s a win. I was hoping she’d grow out of it but a year later she’s still obsessed.

Is the puppy being left outside for long periods alone? Dogs are innately social and need to be with people or other dogs or they get destructive due to boredom. I think a solution may be more interaction with the pup?

And I don't want to sound jerky if that's how I came across-it is wonderful your friend is fostering-I just love dogs!

Pup is probably bored. My neighbor is stuck caring for animals. DD is supposed to foster and place rescues; problem is, she leaves day to day chores for MOM, who’s not keen on doing it but will never antagonize DD by saying NO. I’m out of the drama except to help w/question at hand.