In my estimation, Becca has already prepared Abby to some extent for the idea that she will have to “share” Frank. My primary argument for this is her gift to Abby. “Big Daddy.” Another example of this is that she gave Abby tips on how to get Frank to sleep in bed. Yes, this was all before Frank vetted Abby, but for one thing, we don't know how prepared she had Abby at that point, and for another, we also know that Becca is willing to break promises if it serves the cause.
Yes, there will be some fallout, but I don't think it will be as dramatic as when she found out about Aaron, or Lilly's meltdowns about her ex-boyfriend or her father.
Edit:
However, I can roughly imagine the scenario:
Cut to Lilly, Becca and Mei's apartment, Becca is streaming.
Angry shouting can be heard.
Becca: Chat, don't pay attention, it's just neighbors.
Chatter 1: Doesn't a voice sound like your sister?
Chatter 2: And didn't another voice sound like your boyfriend?
Chatter 3: Also, I'm sure I heard a second female voice.
Becca: Oh, chill, Chat and concentrate on the stream.
Cut to Beth and Vanessa's apartment.
Beth is making a soufflé.
Soufflé: Collapses.
Beth: ...
Vanessa: Sounds like Abby figured out what you guys are doing Frank. What are you doing, honey?
Beth: Getting an ice pack ready. Just in case he'll need it.