Possibly a dumb question, but is this person sure their hay is "good"? A healthy horse does tend to avoid eating hay that is off. I wasn't sure from the post if the mini was getting quite a bit of hay and wasting, or was really avoiding any of the hay it was being given. If it's avoiding any hay it's being given, the hay could be bad, or the horse could have trouble chewing it for some reason if there are dental issues or an off bite.
And sorry, don't have any knowledge of that complete feed. I personally use soaked beet pulp as part of the way I supplement my hay (though my horses also get a different kind of complete feed). Beet pulp is digested in much the same way as hay, it doesn't take up a lot of space, and it's relatively affordable (especially with minis).