Alexa,
Although I would not call this the absolute cause, the high temperature might be affecting your female in some way. Saltwater does not have as high of an oxygen saturation level as freshwater, and the warmer the water, the less oxygen the water can "carry." This is not much of a problem with other bony fishes, but it can be an issue with seahorses, who have very primitive gill structures. The cooler the water, the more oxygen saturation the water will have.
You should definitely begin to lower the temp in your seahorse tank. Don't do it drastically, as this will be a shock to the animals, but at the very least, turn the heater down, or off all together, for the time being. Putting a fan across the top of the water also helps lower the tank's water temp. Remember, seahorses are mostly low-tropic to temperate animals, so they prefer temps around 74-75!
Usually, the first sign of O2 deprivation in seahorses is lethargy, then gasping. It's pretty obvious with most seahorses when they are laboring to breathe. Like any animal, they are trying to move as much medium (saltwater, air, you get the idea) through their system to absorb as much O2 as possible. So, like us, they look like they're gasping. I've never heard of a horse passing away from O2 dep. WITHOUT looking like this, but it's entirely possible.
I really wish I could see the horse...or get Pete Giwojna in here to read this. If you haven't done this yet, why don't you grab a small air pump w/ an air stone and bubble air in the hospital tank for the time being, and of course make sure the temp is dropping down in the hospital tank as well.
I don't know if this will help. The horse's inability to hitch to anything worries me....you're sure there are no skin lesions around the tail area?? I'm glad the other horse seems to be doing well, but you may want to watch him closely also. If it is an O2 problem, he may be suffering as well, but not as much. Do you have a protein skimmer in the tank? Any kind....I'm looking for something to help increase surface area for O2 absorbtion more than actual "skimming" abilities.
I hope this helps some. I'm really sorry you're going through this.....sick seahorses, somehow, always seem to bother us as aquarists than many other fish ailments. It may seem callous, but it's true. We're all sending happy thoughts your way...let us know how the little girl does! And if at all possible, post pics of the sick horse so we can get a look at her.