Mine disengages when I hit the brake pedal.
Otherwise, I just nudge the HST pedal, and it pops off.
Mine's a 4010.
Test drove a Kubota 7500, and it came off with the brake pedal, too. Bad deal on it, because you can't use the turning brakes because of this.
I suspect the cruise control is only turned off if the correct brake pedal is pushed. Maybe your friend doesn't have the brake pedals locked together and has been hitting the wrong pedal? Haven't checked this or read the maintenance manual to know which pedal does this. Bet it's only either the right or left one that does it.
Ralph