I see a ton of people using the whale eye to really set in that this should be more akin to submissiveness - but I heavily disagree.

A dog this size/age /generally/ won’t show submissiveness in this way.  It’s way more likely your dog is anxious or overstimulated and trying to focus on multiple things at once.  Tell your boarder to try and keep the vibes very chill for a day and only give interaction when interacted with, it’ll give the dog time to pay attention to what they want first, feel comfortable, and then switch the focus to the new humans in their face.  Best of luck to you from an owner of a dog that gives me whale eyes just to ask for food sometimes 😃