Sheesh, I feel like someone should report me!
We have four horses on pasture who don't get fed anything - We were in drought for like 6 years, and they only got one biscuit of lucerne hay nightly when we had a really bad winter. They are all, always hog fat - Even when in work.
If the horse is on pasture, and is maintaining weight, it is fine! Horses are designed to be constantly grazing, yes, but they are designed to be constantly grazing poor quality/small amounts of pasture.
Can you seperate the other horse out to feed? I am feeding my mare now in the mornings, so I just pull her out of the paddock and stand by while she eats, then back in she goes.
Extra grass hay shouldn't make too much of a difference to weight, so if you must, chuck her some hay when the TB gets fed.
A slow feeder haynet is also a great idea - As long as the other horse doesn't steal it!