That's how a re geared DC golf cart drives. You would need to convert to AC or to reinstall stock gears if you want more power.Press the pedal and it will barely drive on trailer. Press pedal and hold it the cart will move as if it has a stall converter. But tops out at 42 mph.
But it’s not sounding like he’s loosing all power as would happen with a bms shut off. It sounds more like insufficient voltage. This is a strange one.
That was my initial thought which is why I didn't suspect a battery problem. Maybe the actual battery cells are the limiting factor, not the BMS, and simply can't supply more current. The rated peak is 2C so if they are using budget cells the batteries may not even be able to supply peak current.