Unfortunately, there isn’t much you can do as long as she has adequate shelter and access to fresh water. I would talk to the owners, maybe offer to walk her or play with her while they’re away?
I don’t like giving strange dogs treats since there are so many sensitivities these days. I know my dog would get sick over your average dog treat. You also don’t want to encourage her vocal behaviors. If every time she began barking or crying she received treats and attention, you could be indirectly creating bad habits.
As sad as it is to see a dog lonely and bored, outside of offering your companionship, there isn’t anything you can “do” to remove the dog from her situation. Simply living outdoors isn’t that rough of a life for a dog, many pups live happy, healthy, fulfilling lives outside, but they do deserve mental and physical stimulation – something they can’t get by sitting around by themselves.