How to I handle a repeat humper?

I have this dog who's generally sweet but he tries to hump constantly. I've had to put him in the kitchen which opens to the garden so he has freedom. He's been fed and has a bed and water.

I'm hoping this is just a reaction to being in a new environment and I feel really bad but if I let him around the house, he tries to hump the humans repeatedly. The owner said he never does this at home. Is there anything I can do for now? or should I tell the owner this isn't working out? any advice would be great