Why is my horse such a butt when it comes to flatwork?

Horses that are used to doing only one thing with a certain person tend to not like doing other things, It could take time and if as you've said he is being fine with your friend it will just be to the point of having to work with him until he starts listening, maybe when jumping try putting a jump up but only go over every couple of laps.. after awhile just slowly work him off of jumping it and make your circles bigger. It's healthy for a horse to have change in their routine cause they can become bored, or will refuse to do other things.