How do I tell a client I don't trust her dog?

Agree with your plan. At pick up, you nicely state what you witnessed and suggest their dog would likely be happiest either in their own home, with a sitter who visits or stays there (without any pets in tow), or in a home where there are no other pets. Let the client know that Rover support can help her find the right match for no additional fee.

I've had these conversations (sometimes not providing every little detail of how the dog acted to prevent the pet parent from becoming defensive and not using any labeling adjectives such as neurotic). It's always clear that while I'm thanking them for letting me care for their fur-kid, that I won't be caring for their pet again.

Honesty is always the best policy, just be tactful when you discuss the issues and I would do it face to face vs. sending them a text which can be misinterpreted. Best of luck

It is not your responsibility to be applying any training at all. Keep the dogs separated or leashed and supervised at all times. Owners can be very upset if certain training techniques are used, and there can be a lot of fallout from training that is applied incorrectly. Even as a certified trainer and behaviorist, I do not train client's dogs unless there has been a discussion about methodology and a waiver signed.

I just had a stay that didn't go well, similar in that the pooch that was at the M&G wasn't the same when he was here without the owner.

At pickup, I told the owner that we had some issues with aggressive behavior and humping. They laughed and said yes, he does get humpy, haha isn't that hilarious? No. No its not.

I did not tell them I won't be taking pup in the future - what is the point? They obviously don't see an issue with the behavior. IF/when they do ask, I will explain that their dog's behavior isn't something I can accept with other dogs in the home. Done and done.