If your family member is a professional equine nutritionist I would be very interested in learning their viewpoint on the fat/fibre feeds. If they are not a specialist in equine nutritional needs I would trust the team of nutritionists that gets paid the big bucks by Purina.
My old pony, who had been thin for years despite my best efforts, to fatten him up, started on Omelene 300 and was up to a beautiful weight in 3 weeks. I attribute it completely the high fat/fibre in Purina feeds, because I tried switching to a different sweet feed and my horse lost all of the weight he had gained while eating almost twice as much. He's back on 300 again.