I think an additional detail could make this a bit more sad, because why not
Grace was planned to survive at least another three weeks, his condition was terrible, but Eridian scientists did believe that Grace was planned to live just a bit longer.
But then Rocky dies. Grace has been spending every moment he can by the Eridian's side, despite the both of them in so much pain. Grace was holding onto Rocky in his biodome, feeling his best friend slowly turn cold inside the xenonite. Adrian is next to them, cradling the two as best they could in the xenonite suit. (I'd be they'd have like, extremely good suits by then, almost carapace-tight) Adrian is singing a beautiful song as they comfort their mate into his permanent rest. Grace is sobbing, loudly. He's never exactly been known to cry silently.
The medical team for the two heros stand a few paces away, humming along with Adrian in their song, now turned mournful and sad. Graces students, Rocky's coworkers, family, everyone was there for the three.
They laid together for hours, Rocky's team only stepping in to document what they needed. Those short moments were torturous for Grace. Watching through the clear xenonite as the team took Rocky, Adrian too, back into the Eridian atmosphere. Once the documentation was finished, Adrian walked back into the biosphere in their suit, carrying Rocky back inside, carapace open to the fog.
Rocky had said in his will (or the Eridian equivalent) that he wanted to be placed in Grace's biodome, on the beach for Grace to be with. For Grace to finally be able to touch his carapace without getting hurt.
Adrian placed Rocky down, Grace taking no time to hold his friend. To memorize every carving, bump, dip, every joint. He cried and cried, Adrian sitting next to him, still singing that mournful song.
"I can't do this without him." Grace manages to garble out.
Adrian could feel as Grace's body weakened, knowingly calling in Grace's team just in case.
The human holds onto his best friend for a few more hours, talking to him, kissing his carapace, and of course, sobbing.
It isn't a big fanfare or deal when the team around Grace hear his heart slow down. When it stops, Adrian just adds a new song to the one they were singing, mourning not only the heros of two whole planets, but the two most important people in his life.
The team does as they need, however doing it into the biosphere, letting Adrian stay right there and mourn.
Eventually, everyone leaves, letting Adrian spend time with their family alone. This time lasts for about a week and a half, Adrian eventually leaving through the air lock.
In front of them was a sea of gifts, Eridian equivalent of flowers, letters, and thousands of childrens toys. Grace was the most beloved teacher on all of Erid, of course there would be gifts. However, Adrian didn't expect such volume. The tall Eridan cries, this time in a form of happiness, for his mate and his best friend, the saviors of worlds.
Adrian goes home, grieves, visiting rocky in the biodome, and checking on the scientists. Grace had asked in his will to be turned into a diamond, the information found on the Hail Mary and given to the scientists to perform it. The diamond is finished, a golden color. Adrian gets the stone carved into their carapace, next to their wedding band. Their blue carapace now decorated with the brown from Rocky, and the yellow diamond from grace.
The world of Erid and Earth both grieve, the two most important people finally able to rest, to relax.
