I think when analyzing an episodic story like 911 it's important to ask ourselves how do the choices the writers make progress the story? what purpose does this scene or storyline serve?
In this case, the questions are: why was it important that Eddie met Abby? why does the audience need to see Eddie's reaction to Abby and to Buck risking his life for her fiancé?
In these scenes Eddie and Sam (Abby's fiancé) play a similar role, they are the person Buck and Abby have moved on with and there are a couple of parallels to both characters (we know Sam is a single dad and Abby has a really good relationship with his children just like Buck with Christopher).
When Buck sees Abby and calls out her name and Abby says his name in return we immediately get Eddie's reaction. When they're taking Sam to the ambulance and Abby says Buck's name, again the first reaction we get is Sam's. What goes unsaid in Eddie's reaction is vocalized in Sam's "you're Buck?" (you're Abby?).
we also get similar techniques used in both scenes, with Eddie's reaction we have Abby still in the frame but out of focus and with Sam's reaction the focus changes from her to him, keeping her in the frame but, again, out of focus.
and now let's talk about framing.
When Buck first sees Abby, instead of having the focus solely on her with a single shot, we get a two-shot of Eddie and Abby. The purpose of this could be to place both characters in the same category of love interests to the audience, the ex and the future partner. Buck looks at both before snapping out of it and asking Abby what she's doing there, almost like he's being confronted with that same feeling too.
As Abby and her fiancé enter the ambulance, Eddie is back in the frame next to Buck, both couples facing each other as the door closes on Abby and Buck. We cut to Eddie and Buck and a final exchange between them setting up a question we likely already have the answer to "what's next?".