Had to do a conversion (guilty of metric!), but 8lbs converts to over 3 kilos, right? A horse's stomach capacity is only two kilos, which means that when your horse is being fed, the food is getting shoved into the small intestine far too quickly, and through the rest of his gut. All the food is going to be moving through the gut too quickly for the nutrients to be taken up sufficiently, meaning that the intended benefits of the hard feed won't be there, which she evidently needs if she is underweight. Also, even if she doesn't already, the large feed will exacerbate her ulcers, as the levels of the hormone gastrin will be increased, increasing his stomach acid levels.
If your BO is not willing to split the feeds up, I would be contemplating either moving (if it is possible, I mean, who knows what else he might be scrimping on in your horse's care?), or asking a friend who has the time in return for favours/a small amount of cash.
I want to wake up and find a world in remission,
Free from the grasp of the human condition.