I'm falling asleep so I'm not going to articulate this well, but Stratt's and Grace's friendship keeps killing me when I think about it too long and I need to get some of my thoughts out.
If the world wasn't ending, they never would have met. She was an admin at the ESA in Paris and he was a middle school teacher in California. Those professions have literally zero over-lap.
But the world is ending, and Eva Stratt is going to save it so in her mission she finds Ryland Grace. And she finds more then just a scientist to help with saving the planet, she finds a friend.
She keeps herself purposefully aloof, purposefully cold, purposefully distant from everyone else on the project. Makes sense, she needs to stay impartial to everyone else as her only goal can be saving Earth. But in spite of all of that, Grace somehow manages to wiggle past every wall she has up, and somehow becomes her friend. I don't think she ever let him know it explicitly, but it's very clear she is, at the very minimum, fond of him. It shows in the movie, with the occasional softened gazes, the small smiles and the "You don't bother me". The scene of them during the demonstration, with him turning to her and wiggling his brows while she grins back at him. The entire karaoke scene, where she's looking directly at him while she sings, and him looking back with a fond smile. Cutting off the song early because she's already shown too much of her heart as it is. Eva Stratt clearly holds a warm spot in her heart where Ryland Grace is perfectly nestled, even if he doesn't fully know it.
Eva Stratt was starting to let down the wall. You could see it in the way her smiles were wider, her posture looser, the way she responds to Grace's silly bow to her. They're about to launch, the mission is almost done. Even if the Earth was condemned to freeze, even if she's condemned to be the planets "whipping boy", maybe she could allow herself one thing. Maybe she could keep her friendship with Ryland Grace, even if she would lose everything else.
But then the explosion happens. In the movie, they jump from the explosion straight to the scene of them having the meeting announcing Grace is the back-up. But in the book, there is a moment where Grace is overcome with grief for the dead scientists but Stratt panics. Grace doesn't understand her reaction as panic, he thinks she's being cold about Dubois and Shapiro being dead. But she's not. She's panicking. For a moment, her emotion is over riding her logic. In the book, her instant reaction is "Cry later! Mission first! We need a new science specialist, and we need them now!" She knows, logically, who her third option is. She's always known this. She has everything planned down to every possible scenario. She has redundancies for the redundancies. He has always been the third option. But in that moment, she still has the knee-jerk reaction of "No please not him. Anyone but him." But he is still her third option. Her last option, now.
And Eva Stratt has to take that last option. Even if it means ripping Ryland Grace out of that warm little spot in her heart. Even if it means ripping out a piece of her heart with him. She doesn't want to kill her friend. You see it in her face. "Please, sit down and we do this differently". You see her steel herself, and shakily nod to herself. Even if it's killing her to kill him, it's his life against the entirety of the human race.
The needs of the many outweigh the needs of the one.