How do you get a stubborn dog to get on their harness/leash?

Sounds like you handled the situation as well as you could. I think having the owner put on the harness before leaving for the day is a great solution. If the owners aren't comfortable with that, it's up to them to provide different equipment. After you've had more encounters with this dog he may feel more comfortable with you handling him, but continuing to push the issue when he's clearly wary of you putting his harness on him won't help him develop any trust.

No, that's exactly what you should do. The owner should have the dog in his harness. Hopefully you won't have any further problems attaching the leash. I frequently leave a dog's harness on all day, only taking it off after his/her final walk of the day. It is not only convenient, but may be where the I.D. tags have been affixed. One of my owners had a harness made up with the dog's name and phone number embroidered on the chest part. If you run into problems attaching the leash, try distracting with a treat or toy in one hand and the leash in the other.