.....the best lie is no lie at all, exept when it does not work. then you lie.
As much as I normally advocate honesty as the best policy, this
really is not the time. The truth causes more problems than it solves, as it comes out.
It's preferable to give an answer that fits the shape of what relationship we're selling. We want to convince them we're friends and how we met, while also providing a connection for why Ruby is living with Homura, because that's fairly likely to show up.
Being old friends who were orphans together fits that image better than, met at a hospital, became BFFs, moved in together. Cause I would have a
lot more questions about how this improbable series of evens came to be, even just out of interest in a good story.
Which is a story we don't want to tell, because Homura and Ruby's true circumstances are so unbelievable, and we'd have a hard time coming up with a more realistic yet still believable basis for this level of friendship.
These girls aren't super-sleuths (well, Madoka and Sayaka aren't, jury's out on this magical Hitomi), but we can reasonably expect them to run into some events. Madoka's nurse officer thing, all of them maybe wanting to visit, or asking where Ruby or Homura live, and or Ruby's family situation...
Orphan backstory frames that better. And gives us more room to fill in the blanks. If our stay in a hospital comes up, we can fold it into the overall backstory as needed.
Presentation matters. Take Obi-wan. Tells Luke Darth Vader killed his father. True, from a certain point of view. But if Obi-wan had just straight up told Luke the truth? Well, Luke probably would have tried something stupid like join the Empire, or try to talk to Dad, and fail at the redemption because Luke can't stand up to Vader when he says get in line boy.
In this case, Obi-wan did Luke a greater service with his creative interpretation of the truth.
And this is dragging on too long.
tl;dr Telling the truth here can do more harm than good. Instead focus on presenting a believable enough backstory that's not outright falsehood, but suits our purpose for this introduction future interactions.