When they knew

Nine ball

Debbie strides into headquarters that very first afternoon they meet bitching about the F train, then asks Nine Ball for her real name. Lou gives her this look and pulls her aside. They argue in hushed whispers behind a pillar, and Nine Ball just manages to stop herself from crowing something about a married couple having a domestic quarrel.
She means it, then, as a joke, but then they get into the real planning of the heist and it starts to seem less and less like one. She cleans up their footprint, and in doing so delves pretty deep into their pasts, their childhoods. Everything Debbie's been involved in seems to tie back to Lou in some way, shape or form. It's really telling.
She manages to pull up search histories and bookmarks from half a decade ago, before Debbie went to prison, before Claude Becker. Her fingers stutter to a stop on the keyboard at some of the stuff Lou was looking at - wedding bands, websites of reputable jewellers, venues. She discreetly checks the timeline - right before Debbie started working with Claude.
Shit.
Nine Ball looks further. Looks into Debbie's footprint during her trial and holding - after she was arrested, before she was incarcerated. There's some stuff that seems to come from an angry place, but also an email, sent from a burner to a now-defunct address. Short and (bitter)sweet.
Danny. I need you to watch out for Lou while I'm in here. Don't let her do anything stupid like try to kill him. I don't want you trying that either.
I made a mistake. I don't want her to be paying for it.
Tell her I'm sorry.
That one was sent and successfully reached the recipient. Nine Ball's heart clenches a little, because she's good at what she does and she uncovers a draft that says the same thing, with one more line added below -
Tell her I love her so much I'm so sorry I was so stupid she was right and when I get out I'm never letting her go ever again I'm so s
Nine Ball wipes that from the internet forever and keeps her mouth shut. She's not the type to go interfering in the lives of people she actually cares about when she has no business to be doing so. She's never been in prison or even been close to being arrested - she's too good at what she does for that - and she can't even begin to imagine what it must have been like for Debbie, five years in.
She and Lou will figure it out on their own, in time.
(She's good at pretending she doesn't give a shit about anything or anyone, but Nine Ball takes a quick screengrab of the email draft before she wipes it forever, keeps it safe on her drive, because Debbie is bold and takes risks and Nine Ball worries. She can see Debbie taking too great a gamble and losing, and she can see Lou being left behind. She thinks it might be for the best to make sure this, at least, survives. One tangible, true record of how much Debbie Ocean loved Lou Miller, even if everything else ever fades away.)