I'm sorry, but the seahorse didn't die for NO reason. First, it could have died from the induced stress of tankmates not suitable to it, especially the gramma and the ocellaris. While occasionally it works, most times it doesn't, especially once those fish mature.
It also may have died from parasitic infections that passed from those fish, and the seahorse, not having grown up with exposure to those particular parasites, succumbed to their invasion.
Tank conditions most likely are not suitable for the best chance of success for seahorses. Their immune systems are severely lacking when compared to other marine fish. Like people, some seahorses may have a strong immunity for their species (still nowhere near other marine fish though) and others have virtually none, with the rest falling in between.
Worst case scenario would be the poorest immune system coupled with tank water that enables nasty bacteria to expand, with temperatures that enable it at a faster rate (exponential scale not linear, especially above 74°F) and perhaps a small wound due to tankmate, and further reduced immunity due to stress from faster moving fish.
In any case, at the top of this forum there are a lot of stickies that she can read and glean necessary information from, and if she has any questions then, post a new thread to get answers.
Also, Facebook has seahorse forums like Seahorse Sources Group and Seahorse Solutions Group and on the main page both have a "Files" listing under the headings and contain a lot of great up to date information.
A last point for now, best chances of success come from buying captive bred stock direct from a reputable breeding source like Seahorse Source or Seahorse Savvy.