Help getting weight on

I have a friend that has this paint mare that has been a TOTAL pain in the butt to get weight on. Now she is not dead skinny just not as fat as my friend would like her. This mare has one big strike against her and that is she has the longest back that I have ever seen and that really make her look a lot leaner than she really is.
Here is what she is fed twice daily

2 (3 quart scoops) *sorry can't weigh my food as my scale broke and got to wait til payday for another one*

Southern states Triple 10 feed --- 10% protein , 10% fat, 10%fiber ( I also mix in a little oats cause if you don't she wont eat it)

She gets plenty of hay twice a day. This mare can't get alfalfa of any kind from what I have been told it gives her the runs really bad.

Hi,
Can't find any info about the 'Triple 10' feed except that it's a high carb, high fat, low fibre sweet feed. Therefore that could be part of the problem. More info on the feed would be helpful. Feeding a lower carb, higher fibre, healthier alternative could be the answer. Feeding any 'hard feed', but particularly carb rich food little & often is also important.

If the horse is not thin - just that your friend likes fat horses, perhaps it would be better to direct her to sources of info on healthy horses, weights & feeding? Is the horse in hard work? Are her teeth in good order? Has she been scoped/treated for ulcers? Worms?

I would definitely get the teeth checked out. My rescue TB was terribly thin when I got him. I got his teeth done and added a bucket of Purnia's Amplify for weight gain and it was amazing to see the difference in the weight change in 2 months. But also, despite what you do, some horses, just like people, will not gain weight.

Has she been wormed? A worm count will tell you what wormers you need to be using. You can't over feed good hay. Also feeding the same amount over more meals allows them to diest it better taking more out of the feed they eat so try giving her 4 small meals, adlib hay and worm if needed. I msure she will be big and fat soon. But with a long back being a bit lean is better for the spine. It puts less pressure on.