How do I talk to an owner about there pet using the bathroom in my house and chewing things constantly?

Did the owner tell you the dog was house trained? Did they provide any tips on how to keep up any training they had done? Accidents and chewing are to be expected from a puppy, but if the owner has been doing any training they will have hopefully provided you information on how to reduce these things from happening frequently. I would message the owner and explain that puppy has been having accidents in the house and not going when you take her out. Ask if they have any tips or tricks that have worked for them. You can also mention that she's chewing a lot - maybe it's anxiety, but more likely it's just being a puppy! Ask if they have recommendations for chews/toys/bones she could use.

Unless the owner promised that the dog was 100% house trained and didn't chew, I don't think you should expect anything but advice from the owner. As Moureene said, this is why most sitters charge more to watch a puppy! Best of luck!

Puppyhood was probably one of the most difficult years of my life with my dog. He was a maniac. He never slept, didn't listen and would wait until we got inside to go potty. He was... a puppy. This is one of the reasons some sitters charge extra for puppies. They are EXTRA WORK. It's like taking care of a baby as opposed to an 8 year old. It's also a reason some sitters don't take puppies at all.
All of the behaviors you are describing are normal for puppies and they can be frustrating. Just like it takes a while for toddlers to potty train, some puppies can take a long time before they 'get it'. Hang in there! :)

Well realistically it doesn't matter how long you leave her outside. When she pees, she will need to go in another couple hours. Puppy can hold it (if they've been trained) about the amount of hours to month. So if the puppy is 5 month old they will need to pee every 4 hours. If they are playing and walking around the house that same puppy will need to go every 3 hours. If they have just eaten food they will need to go outside after 30 minutes.

It's important to tell the owner to bring their toys and for you to have toys also. I have watched dogs from 2 weeks old to 20 years old...... every single dog chews. It's not until they get a lot older AND have also been trained that they know what things to properly chew on.

I'm sorry the little gal is giving you such trouble, but what you're experiencing is completely normal behavior for dogs and especially for puppies.