How do you stop a supposedly "crate trained" dog from barking at night?

It happens. Some dogs have never been boarded before. Some are just scared and anxious. A dog can be crate trained and sleep all night with no problems - at his own home. But now the dog is scared, away from the owner and wants to be close to someone. That's YOU :) I have experienced this too when boarding dogs. Some dogs adjust just fine, some are really stressed out and will bark all night until you give in and take them to your bedroom with you.

In addition to the advice already given, maybe try placing the crate as close as possible to your bed with a comfort item (scented from home- might be a toy, blanket, or clothes). The following advice is for the future: Always strongly encourage the pet parent to book a minimum of one overnight stay before a longer stay like this, for everyone's benefit - the dogs, the owners, and yours.

That can certainly make for a rough night - I feel for ya. I have a dog that I watch very often. He LOVES it here and gets along with all other dogs and human visitors. However, the first night of any stay he is almost always up all night barking. He knows the place, and is comfortable, and he usually has something from home that has his owner's scent. It's just how he is. If he arrives early enough in the day, I try to play a LOT and exercise him quite a bit so he's super tired by bed time. That seems to help. I like the recommendation of trying to just put the crate right near your bed and put something in it with his owner's scent. Also, I'd say go ahead and very nicely contact the owner to see if they have any suggestions that work well to calm their pup down. Good luck - I hope the rest of the visit improves.